I’ve been adding docks since the product was released (NH62 at FS.to, 52B most recently). So the answer isn’t “Can’t” it’s “We need to add docks”. That’s what confused me.
And, I believe there are some default seaplane bases that have docks. Check out the Kenmore docks in Seattle.
They have been adding parking spots now to backwoods airports. For instance ME22, a very small private strip in Maine, has spots interestingly, with no plane outlines to support their existence. Too many spots for a strip that size, in fact, lol (I’m about to release a redo of it. I kept the extra spots, cause, well, who cares 
). So, nothing stopping them from adding dock spots with AI as they did here on land.
I like that they are reacting to our inputs