.... Oh. Okay now that explains a lot. Ritsuka being from a nation that's basically just a big island, she's not used to thinking of people as not knowing how to swim in the first place.

"Oh, in that case, you probably don't want to get started in the ocean, no. Maybe a pool?"

Assuming he wants to try to learn to swim, that is. But, back to the present...

"You can still wade a little though without having to worry about sinking or anything. If you walk in the middle of the wet sand, you'll get waves that touch your feet, but nothing deeper than that."