Yes but they still have interior and exterior moisture, and evolved from plants that weren’t always in a lake. And I suppose occasionally grow in temporary bodies of water.

Surface area is evaporative exposure and frost exposure, might not matter much to temperate acclimated plants but it may be an evolutionary holdover. Also plants don’t really expend energy to modulate their bodies. It’s more about moving water around their internals.

Moving may also prevent some fungus or algae colonization.