By the way, the Basic rules already have the tool, though not in the Tools section - "fishing tackle"
On the surface, have that proficiency allow him to get enough food whenever they are near a body of water with fish.
Halibut you also give him proficiency with net and trident. Those are fishing tools.
On the net, I'd say gill the character a couple of flavorful perks, but nothing titanic.
Thausubmerge.
As a weapon, I'd call it improvised or use the closest approximation of standard weapons.
I have never seen anyone flounder around so much for bad puns. You oyster be ashamed of yourself.