In the episode where spongebob finds a submarine they start selling food on the rode.When they get chum dropped on them many holes get punched in the sub,which means water comes rushing in...Wait,say what?They are under water.How does water come rushing in when it is already filled with water?
How? Because it is a kid's show? There are LOTS of water underwater incidents in the SB universe. Think of the Bubble Buddy visit to the "beach" episode, or any time the "cast" goes to teh lagoon with Larry.
Most of the things on the show wouldn't survive underwater. Like, fire, paper, electricity, hamburger buns, paintings, etc...It's a cartoon and it's not meant to be completely accurate, it's meant to be funny