Agree to Chris_Byler: summons/transforms are probably better. Sea snakes are excellent example. Considering turtles - I believe just the name of snapping-turtle is worth it.
And crocodiles.
Considering hyppopotams - I remembered them initially because the name means "sea horse" in Greek, so they can be freely hybridized with denisens of ancient bestiaries...
As for mantas - now I think that I see where they can be placed - they can be a theme of sadly absent LA Oceania!
By the way, spells which actually changed terrain type were among my favorites in old Master of Magic, but I'm almost sure that they can't be implemented under current engine. Which is a pity - gods and god-like magicians should have an ability to move or raise mountains...
