I think I have an 'idea' for the magic system. Like, instead of commanding elemental powers, you control sea creatures through magic objects. Like you could command normal fishes through a small, easily enchanted flute. Dolphins with a horn passed on from generation to generation. Eventually there would be relics from a long lost civilization of mer-people (Atlantis?). Those relics could summon ancient beasts from the depths like a Kraken, Leviathan or whatever they call that guardian of Atlantis in the Disney Animation? Again, it's just a suggestion.