*takes a peek inside RC*

[Aix]

 

All other WATER monsters you control become the same Level as this card. When this card destroys a monster by battle: You can add 1 “Shark” monster from your Deck to your hand.

 

[Toffee.]

Should be a Fish, IMO.
But I suppose you don't want this to be searchable by Mermaid AND Beautuna, on the off chance it can get out of hand.

[VampireofDarkness]

Rank 4 for Days! I suppoose this is a very interesting card, especially thinking about Mask Change Second. Knowing Mermail and how they love to Special Summon, you could just keep spamming and Rank 4ing, while if it gets targeted, you can just Mask Change into Acid at any time and just blow up all the backrow. I suppose it wouldn't be consistent since it isn't quite as accessible and the deck can't run Shadow Mist, but it is a thought. Their probably are more than a few consistent engines though that could get this card quickly fit into a WATER Deck as a staple along with Lemuria, which would lead to Level 5, 6, and 7 plays as well. Thinking about Sharks, I do feel that it should be a Fish-Type so Mermaid could get it and there would be more synergy with the entire theme. The last thing I would complain that it should have "Shark" in its name, but this is the only card that grabs any "Shark" monsters instead of just Fish-Type so I suppose it leaves that out of the question.