i think it suspected it would fall on water but wasn't smart enough to "check" if there was water underhim to begin with.
Not like it would had worked anyway, since falling from such height, regarldess of the hat, would make it sunk for a second in the water before resurfacing, which for a yukkuri is pretty much being dead already.
What i meant by being dead already is that they would be in the very water, instead of over the hat, the hat would probably had filled with water and sunk, or be too wet to remain on it's shape.
So the yukkuri would remain floating in the water while asking for help and melting. Remember that Marisas NEVER dive bomb with their hats, and that's probably because it would lead to a sure death, they usually put the hat in the water, hop in and start sailing.