Since digestion requires water, it’s probably better that you not eat unless you have an adequate supply of drinking water. The food sources available in the ocean are fish, plankton, and as a last resort, cannibalism.
In order to catch fish, you need access to some type of fishing equipment. You can fashion lines from any rope or string (eg. shoe laces). If you have a knife, an aluminimum can may be used to create shiny hooks that attract fish.
Pull in any seaweed that you come across and look through it to find edible fish, crabs, or shrimp.