I'm not sure about not using the Genex Undine engine, I don't seem to like it that much. I do like the Mermail I put in, seems it could go well with Seahorsemen. I also saw another card named Abyss Soldier which it seems could go well with this deck. I also left out some cards like Surface, I already have Atlantean Roar, so I saw Surface as not that useful and the last card I didn't put in the deck was Pot of Duality, but I think it might be useful, although the deck is based in special summoning a lot and otking with the Deep Sea Diva + Genex Ally Birdman combo.
Let me know what you think about the deck, please, and what changes i could make to it. Thanks.