Everyone wears a wet suit, because the water is cold, and the wet suit keeps you on the surface. You can dive down, but only for a few seconds before you come back to the top.
If you want to practice for the cold water, take a shower or bath, and don't thru the hot dial. That's what it's going to feel like when you get in the water at DC. The good news, they have a lazy river and that water is heated.