It looks like a realistic tuna can that is found in a real-life shop. It comes in a circular green can with the food's name inscribed in purple text within a turquoise oval decorated by yellow and orange flowers.
The can of tuna is mentioned by Mr. Krabs when he tells SpongeBob and Patrick that they would either end up inside of one or be sent to the Gift Shop if caught by the hooks. After this, a close-up of a realistic tuna can is shown both times, one time with SpongeBob inside the can.
Patrick ignores Mr. Krabs and continues to play on the hooks, resulting in a fisherman catching him and ending up in a can of tuna at the end of the episode, asking for a can opener.