Has anybody mentioned a diver yet? The whole reason Buggy didn't want to eat the devil fruit was because then he wouldn't be able to get to the treasures deep in the sea. If my memory is working today. Nami would be in favour of a diver, at least.
I envisage a merman with a harp as a diver and musician. Swimming around the ship, scouting the ocean floor for obstacles and treasures all the while playing tunes for the rest of the crew.
A young and superbly skilled merman diver who dreams of finding his legendary mentor who vanished a long time ago (implicated connection to Gol.D Roger) or something like that and unites with the Straw Hats to pursue his dream. He will first have to deal with his problems in Mertown, of course, and Luffy helps him out.
The other mermen laughs at or shun our poor merman when we first see him, but Luffy shows them the power of dreams by doing something awesome that change their lives for the better, shutting them up and inspires our merman to follow his own dream.
Also turns out that he has some amusing personality quirks, and named special attacks. He is friends with fishes, and constantly tries to prevent Sanji from cooking them and Luffy from eating them. He is distrustful of Chopper because he's half human and half animal, which is ironic because he is half human half fish himself. Nami will constantly hector him about finding valuables at the bottom of the sea. Those things will be the running gags of his character. His weapon of choice is the Trident, which he uses along with Tail-Kwon-Do, the merman martial art form.
:D
Would you accept him/her as a character?