You may be somewhat of a fishy person already, but not everyone goes this far. You have developed the techniques of seafoods, and have somehow worked them into your line of work.

Salting Seafood

At 3rd level, you can now do some basic cooking. You are now proficient in cooking utensils when cooking any seafood. When you cook seafood that has been seasoned with something basic like salt, whoever eats it gains 2d6 temporary hit points. This increases to 5d6 at level 11.

Fish Slap

Starting at 3rd level, you can use fish as an effective weapon. You become proficient with it as a weapon, and they do 1d4 bludgeoning damage. Once per long rest, when you make an attack against a creature with a fish, you can add your average sneak attack damage (Maximum roll / 2 + number of dice rolled - 1).

This increases to twice per short or long rest at 15th level.

Think Like a Fish

Beginning at 9th level, you can think like a fish. You gain a swimming speed equal to your walking speed, and can hold your breath for an additional minute.

Bad Smell

At level 13, you can create a minor magical odor. This smell works in a 30 foot radius, targeted 20 feet away from you. All unsuspecting creatures must make a wisdom saving throw against 8 + your intelligence modifier + your proficiency bonus.
On a failed save they go to check out the smell's origin, gain disadvantage on saving throws and attack rolls, and take 1d4 poison damage when within 10 feet of it. Nothing happens on a success.

This can be used once per long rest.

Big Splash

At 17th level, you can now make a big splash! You can choose one of two effects once per long rest with this.

Tsunami: You can cast the spell Tidal Wave, with intelligence as your spell casting ability. The spell also deals an amount of additional damage equal to your sneak attack bonus to one creature hit by the spell.

Supreme Swimmer: You mimic the fish, to its most minimal movements. You can do this for 1 minute, allowing you to swim up water falls at your walking speed, and your swimming speed becomes 80 otherwise.