The S.S. Diarrhea is only mentioned by Mr. Krabs in the episode, "Squilliam Returns."
Description
It is a boat where Mr. Krabs was the head chef. Little is known about it other that Mr. Krabs worked there and that it presumably sells horrible food.
History
Squilliam Returns
Mr. Krabs was asked by Squidward if he served on a restaurant called the S.S. Gourmet which he said he did. He later reveals that he mistook Squidward for saying S.S. Diarrhea and that he cleaned the bathrooms of the S.S. Gourmet.
Pull Up a Barrel
It is very likely that the ship Mr. Krabs was on in "Pull Up a Barrel" was actually the S.S. Diarrhea. However, Mr. Krabs knew how to cook food on that ship; though, the food probably caused cramps and diarrhea, hence the name.