polymorph into Acquatic creatures

[Dyslexic]

Hi, i need some suggestion about the creatures I can use to be strongest into an aquatic dungeon.

I have a gish (JPM+Abiurant champion) lvl 12. I'm an outsider and my caster lvl for polymorph is 14/15 (I'm not sure, sorry).

Thank you very much.

[CMagnum]

I found this link helpful for the polymorph spellhttp://brilliantgameologists.com/boards/index.php?PHPSESSID=vb8379fugta0rbe434qs0omco1&topic=10479.0