Based on the webtoon
Siren's Lament.
A is a siren, an underwater seeker of broken hearted and lovelorn people. They lure them into the sea, calling them with promises of a new life and an escape from the pain. Upon agreement, a contract is made and the human loses their memories, gains a tail and is curse to wander the seas until they can pass on their contract. The Siren gains legs and a new chance at human life.
B encounters A one night, lovelorn and depressed. They get lured into the contract but change their mind halfway through. Instead of the curse being passed on, it is shared between them. A now has legs but both need to return to the sea every so often to retain their health. Somehow, they must find a way of lifting the curse off themselves.
I don't have any preference on which role I play. MxM, MxF, FxF, I don't mind. Take your pick! Requesting romance on this one please, since the plot centres so much on love.