Cut My Tail, Keep My Crown

Merman Marn saved a captain, who offered 1 of 3 daughters as bride. He loved eldest Clarissa. A jealous witch cursed them into monsters. Marn traded tail & magic for legs & cure to save her. They fell for Lucas and abused Marn. Desperate, Marn pacted with Amphitrite via old relic. The curse backfired; they repented, finding Marn their salvation.