Well, elemental powers would be kind of useless, don't you think? I mean, what's air or fire going to do underwater? It feels as if water is the only useful one there. If you'd like i could think about it some and come up with some suggestions.

Further wouldn't it be much more interesting that there is just one 'ocean kingdom' where all sorts of sentient sea creatures can live? (like mermaids and mermen, but also half-octopusses (looking at your there, ursula)). Who then, for some reason, go to war with the humans living at the coast. So people could chose which side they take? (humans having 'advanced weaponry like basic gunpowder muskets and cannons and mermaids having magic)? Again, it's just a suggestions.

(ps: would you make it a free or a casual RP?)