Also, try clicking up and down on the upgrade, so that the unit changes ship multiple times, without clicking OK. Ofc, no money is drawn, but after four clicks, the "OK"-button goes red and can't be pressed.
Edit:
I tried clicking end turn a few times, saving up gold (6 every turn). And it turns out that with more gold, it takes more clicks until the "OK"-button goes red. Right now, 11 clicks are needed.
Edit again:
I haven't been able to try it out on river docks yet. Only ordinary docks.
Last edit:
Fixed everything into position. Now you can look a both bugs without having to embark disembark move etc!