[Chained Birds]

Mermails/Atlanteans are currently the best water themed deck. They revolve around the discarding of Atlanteans to fuel Mermail effects to gain massive +s out of every turn. Potentially one of the reasons why Brionic (The Ice Barrier Lvl 6 Synchro) was hit by the ban hammer, though he still deserved the hit for other reasons...

Another Water deck, that I use for fun, is a Nimble Frog deck. It revolves around Swap Frog and Nimble Angler with the use of a field card (Can't remember the name at the moment) that can upgrade the levels of water monsters you control by the number of water monsters you control. Essentially, it turns a field of Lvl 2 weaklings into Rank 4-7 Monstrosities!

Next is Sea Lancer. The deck revolves around the frog engine of using Swap Frog to fill up your graveyard and Ronin Toadin's ability to summon itself with the removal of another frog in the grave. These guys work together to make the monster Sea Lancer effect really useful, in that he can equip the banished frogs to himself and recycle them back to the grave while gain +s. It is a bit confusing to explain via writing and usually needs a demonstration, so try finding a YouTube video and see what ya think.

Lastly, Gishki. The former best was the Gishki Hieratic hybrid which was made infamous for ruining the opponents hand first turn, and ending with some big beaters. But due to the Lvl 6 Gustkrake getting hit, the new top Gishki is the Zeal Gigas Cannon deck. Running Trageodia and Zeal Gigas, the deck strives to get two Zeals to the field, use their effects to draw and potential clear the field, then attacking for 6400dmg, before XYZing into Gustav Max and using its effect to deal 2000 damage to the opponent for game. It is also mainly a fun deck, but if you like massive four-armed Fishmen with over 3000 Atk with a pretty nice eff, then the deck might interest you!

But those are my top 4 picks for best Water deck. Other duelist opinions may vary.