I'm currently using this water Deck supported by Mako Tsunami. I find this extremely good both aggressively and defensively. It also boost your Life Points using the card "Cure Mermaid" supported by "Umi" and/or "Wonder Balloons" cards.
I use the "Wonder Balloon" for emergency purpose to lower the opponent's attack points drastically. "Kaiser Sea Horse" has a starting attack points of 1700. That card is a Sea Serpent type that would benefit from the "Umi" effect, gaining an additional 200 attack and defense points, making it a good 4 star tanking card with an attack power of 1900.
I run water deck and while this isnt bad. Im running a better deck. It uses suijin with 20/20 for pvp reasons. Hit platinum in under 5hrs on mu 2nd day of the game