Thunder king Rai-oh can be used to stop the deck from searching while essentially beating over all the lower leveled ones and it can stop synchro and xyz plays. Deck lockdown can be used if you want the deck to not be able to special from the deck. Debunk, Necrovalley, The End of Anubis, and Soul drain can stop their grave and/or hand effects. Macro cosmos and Dimensional fissure can stop their effects that discard to the graveyard. Fossil Dyna, The Vanity cards, and jowgen can be used to stop the deck from special summoning. Night Beam doesn't hurt the deck really but is a nice card if you are afraid of abyss sphere.

As for decks with good match-ups E dragons and prophecy obviously are a good match-up. Evilswarms also have a good match-up due to the backrow it runs and also ophion which stops megalo, abyssteus, synchro plays, etc. Grave keepers also have a decent match-up due to necrovalley and the backrow the deck runs. I also will say hieratics have a decent match-up due to the fact hieratics can otk more consistenty and faster than mermails ( from my experience at least ). That's all I can come up with at the moment but yea those are cards and decks I believe have a good match up based on playing with the decks, regional reports, watching duels, theory, etc.

Macro is indeed, as mentioned, a superb counter against Mermails, as is Dimensional Fissure. 
Another viable option, especially in a mirror match, is Abyss Dweller. It's effect of preventing effects to activate in the graveyard kills all Atlanteans as well as Linde and Gunde. Due to the 500 ATK boost it gets if one of the materials is water it goes up to 2200 ATK which is pretty beefy. 
Another deck that can make good use of that card is Gadgets, as they run Tin Goldfish and therefore can too, make a 2200 ATK Dweller. 
Debunk is good for obvious reasons, as is Mind Drain. 
Mind Crush is another hand control option. Teus and Megalo as well as Tidal activate in hand, so when they activate you can chain a Mind Crush and call that specific card to prevent it beeing summoned and getting it's effect. It does not prevent the discarding for the effects though, meaning you might still need to deal with searches and pops.

Thunder King is not a good card against Mermails, in my opinion. Yes, stopping searching is good but ultimately Rai-Oh is too easy to get rid of, be it by a discarded Heavy Infantry or by simply beating over it with Megalo, Leed or Tidal. Plays which can be easily accessed with Speheres and Gundes. 

A deck that should have an advantage against Mermails is Macro Rabbit. Macro prevents the discarding effects and summons of big things whilst Laggia prevents other things from going of on the field, be it a Diva or Sphere play. 

With all the Macro-Love we are having here I think it is important to mention that, as good as it is, it might not be an option this format as many decks side Imperial Iron Wall to counter Dragons. Mermails side it too, simply because they benefit from it in two ways, even though it does prevent Tidal plays.

If you are using Firefist's its more ideal to use Dimensional Hole  Using a monster with higher attack then abbysgaios and having dimensional hole and or macro is pretty much GG. If you add skill drain to that your gonna be unstoppable . Well tried my best

well i thoguh abotu for awhile and the onyl answer i could come up with is royal decree. mainly cause the mermails that i have faced relyed alot on the traps to pull of some of there main combo's. I tend ot mainly see the main mermail trap used ot get out abyss or etc. Ik cards like mst can get rid of a set abyss trap the moment it is set but royal decree cancels out there traps completely.

for monsters the only thing i came up with is shock master since you detach and call monsters so there effects are negated(still tryign to figure out if shock master negates on field monsters or monsters on field and in hand)

A good card for them, Thunder King Rai-Oh.
"Neither player can add cards from their Deck to their hand except by drawing them. During either player's turn, when your opponent would Special Summon a monster: You can send this face-up card to the Graveyard; negate the Special Summon, and if you do, destroy it."

And a deck that I think would go toe to toe with them would be: Elemental Heroes.

Now before anyone bashes it, think about it. Mermails are what? Water attribute. One hero, super poly, and you have a Zero. Zero can help out a lot because of his effect. Also, miracle fusion, have Shining. You can always recycle your monsters to keep something strong out and keep their field wiped.

Zero and Shining are vital for field wipe and power boosts and recycling monsters. I really think that newer Elemental Heroes decks would do well against the mermails. Not traditional, sadly.

Nottu, the Evoker wrote:

 You mean "Dimensional Fissure?"

 Yes, yes I do  couldn't remember the second name and couldn't search it up.

here's a hidden gem vs mermails TRAP STUN on their abyss sphere...not only does it negate the summon of linde which would bring out megalo or whoever respectively BUT trap stun also causes Sphere to miss its destruction SOO the effect of being locked out from spell card will be applied  endlessly until its gotten rid off some how  i've even looked it up to verify this from several sites as well as metal and soul drain is a good 1 2 punch to stop and lock out their monster effects...any deck that can make use of macro and fissure will work very well  vs the deck in question however these have been mentioned already so ill go a different route ive noticed chain burn has a very good match up vs mermail since it can typically stop any otk attempt with a stall card  if their field gets to big lava golems will shrink down those megalos and abyssgaiosand memails will never side in decrees due to their reliances on sphere and other traps

Single card answers to Merlanteans are: chainable s/t destruction (MST and Dust Tornado) to hit Abyss-Sphere as it activates to fizzle it. Macro Cosmos/Dimensional Fissure prevent the Atlanteans from activation or being ditched for the Mermails. Cards that stop Special Summons are also effective though be careful that Abysspike + an Atlantean can clear these out without some other protection. Cards like Jowgen, Fossil Dyna, etc.

Deck answers, ignoring the most obvious choices (Macro Rabbit with Macro + Dolkka and Verz with Macro + Ophion), an effective counter is Hero Beat. While Merlanteans are capable of charging through quite a bit of backrow, it tends to be much harder to get through 4-5 card backrows. Additionally, Miracle and Super Poly give access to Absolute Zero, which not only beats over most of their monsters (except a Gaios with xyz material) but when its removed from the field it destroys all their monsters, which is very useful against a monster heavy deck like Merlanteans. Hero Beat can also easily side several of the above mentioned cards, like D Fissure.

clairedestroyer wrote:

Single card answers to Merlanteans are: chainable s/t destruction (MST and Dust Tornado) to hit Abyss-Sphere as it activates to fizzle it. Macro Cosmos/Dimensional Fissure prevent the Atlanteans from activation or being ditched for the Mermails. Cards that stop Special Summons are also effective though be careful that Abysspike + an Atlantean can clear these out without some other protection. Cards like Jowgen, Fossil Dyna, etc.

Deck answers, ignoring the most obvious choices (Macro Rabbit with Macro + Dolkka and Verz with Macro + Ophion), an effective counter is Hero Beat. While Merlanteans are capable of charging through quite a bit of backrow, it tends to be much harder to get through 4-5 card backrows. Additionally, Miracle and Super Poly give access to Absolute Zero, which not only beats over most of their monsters (except a Gaios with xyz material) but when its removed from the field it destroys all their monsters, which is very useful against a monster heavy deck like Merlanteans. Hero Beat can also easily side several of the above mentioned cards, like D Fissure.

 This is what I was trying to say when I posted about the Heroes. I run Heroes and I know how they would use my opponents monsters as material.

Destroy either their spells,or trap as long as they don't get field control, We can win without difficulty, and the card that can beat them, Heavy storm,Dust tornado, and i think Macro decks can beat them.

Blazer Slater wrote:

clairedestroyer wrote:

Single card answers to Merlanteans are: chainable s/t destruction (MST and Dust Tornado) to hit Abyss-Sphere as it activates to fizzle it. Macro Cosmos/Dimensional Fissure prevent the Atlanteans from activation or being ditched for the Mermails. Cards that stop Special Summons are also effective though be careful that Abysspike + an Atlantean can clear these out without some other protection. Cards like Jowgen, Fossil Dyna, etc.

Deck answers, ignoring the most obvious choices (Macro Rabbit with Macro + Dolkka and Verz with Macro + Ophion), an effective counter is Hero Beat. While Merlanteans are capable of charging through quite a bit of backrow, it tends to be much harder to get through 4-5 card backrows. Additionally, Miracle and Super Poly give access to Absolute Zero, which not only beats over most of their monsters (except a Gaios with xyz material) but when its removed from the field it destroys all their monsters, which is very useful against a monster heavy deck like Merlanteans. Hero Beat can also easily side several of the above mentioned cards, like D Fissure.

 This is what I was trying to say when I posted about the Heroes. I run Heroes and I know how they would use my opponents monsters as material.

 blazer u use heros?? we gotta duel

So i think Macro decks could beat Mermails/Atlanteans

Meh, i dont know why they're good, i just know that they're good cuzz my friend is playing macro decks.

Messenger of peace will give mermails-atlanteans hard to atttack as we know mermails/atlanteans have high attack cards so messenger of peace will have mermails-atalnteans hard to attack, level limit area - b can help too, how bout Dimesional Fissure?, well mermails-atlanteans send cards from hand to graveyard right?, well now they send it to banish, and for Messenger of peace because we drain our LP for 100 each turn we could use Soul taker it can destroy an opponent monster and increase our LP, and we could use Dian Keto the cure master, and cure mermaids to help us gain LP, and for the deck how bout e heros?, they have Elemental Recharge, and how to beat them with e heros?, Well just attack them but messenger of peace block both players attacks and e heros is playing in high atk power, so they cant attack but there's still a way to make our monsters can attack, but i still need to find a card that can negate the effect of messewnger of peace for our monster, and with Dimesional Fissure in our field and messenger of peace we still can add clear world to the field, clear world make mermails-atlanteans discarding cards from their hand to the graveyard but with dimensional fissure they will go to the banish, because they go to the banish they cant use their combo.