Underwater water.

This is a Brine Pool. It's a hyper saline water(very salty) that's about 5x denser than normal water so you get puddles of water underwater. SO whenever you're watching Spongebob and you think think to yourself "How is he taking a bath underwater?" Well that's how dammit, do you ever hate on Spongebob again. These are common in the Gulf Of Mexico, and well in Bikini Bottom.