I'm no expert on carbon, but because it's such a close-range weapon, I'll highly recommend Ninja Squid and Swim Speed up. You'll be able to close the distance with enemies unnoticed and assassinate them in the middle of a firefight! Swim Speed up, of course, negates the lower swim speed you get from NS. I believe it takes 2 mains and 1 sub (7 subs in total) to negate the speed drop.
You can vary your Slosher build based on how you like to play it. General abilities include Ink Saver (main and sub), Ink Recovery Up, and Swim Speed Up (can help with zoning). If you like your tenta missiles a lot, you can run comeback, Special Charge, Special Power up too. Some will recommend sub power up + ISS if you like suction bomb play. I also recommend Bomb Defence up -- because the sloshing animation can lock you in place a bit, you might catch quite a bit of splash damage from bombs, making the ability helpful in staying in a fight better.
Tri-Slosher I've never really used, but it's probably the same deal as Sloshers, but there's a bit more emphasis on your special since it's so damn strong. Bomb defence is also less necessary since you're a lot more mobile and can more easily get out of the way of bombs.