He can swim even he walks at the sea

Its weird because both of you are right.
He CAN swim because he lives and walks under-water, DUH!
And he can't swim because in one of the episodes, I think it was about cramps in the sea (XD) he tried to save patrick, but couldnt swim there

How the hell does a SEA CREATURE not swim?!

that is ma question too! how in the devils name how can he not swim he underwater living in an pineapple! and you ask can swim?!

so true wat the hell wat kinda sea creature cant swim wat a dumb ass but he is funny

It's actually both.
He could swim because he's under water, and he can't swim in the Goo Lagoon.