With practice, one knows where the wheels are, but floats/skis can be a guess given certain conditions.
Happily, for most of the planes I fly, I know
exactly where the wheels or keels are. However, the key point, is on glassy water or unbroken snow, you really do not know where the whole plane is!
If you are higher than you think you are, you'll likely drop it on, though might save it with a whole bunch of power when you realize. If you're lower than you think you are, slamming it on is very likely. A wheel or ski plane might survive this, but a floatplane/flying boat will not - you're going over (whole or broken).
I can assure readers that over glassy water, you might not be able to judge your altitude to within 20 feet. This is why demonstrating glassy water proficiency is a requirement to earn a float rating. There are tricks, but sometimes you just have to handle it with your skill.