He does know that water is heavy - full water barrels are often the only thing Tatooine's winds can't knock over, and water on ships is rationed (generally) because they add to fuel costs. He's never experienced truly wet clothing so he can't really work out how heavy that would be, and he's seen things float on water and broth. So he's quiet while he works out how much of a problem it could be. "People don't float in water?"