It could be related to conservation career, I never tried that career before.
If my sim is on the lot, it allows me to take hold of it (in their inventory) and drag it to the water but you got to find a spot showing green under the boat, if it shows red then the water is not deep enough to put it in that spot. I don't use the docking ladder anymore, I just use the regular one only because they waive every time when they get out of the boat, like they can't go there and I have to click on the deck to get them to go there. Using a regular ladder, if they are on the boat, at least they will sail home and automatically use the regular ladder. (in case you need this info once you figure out how to get the boat out of their inventory, LOL).
If this don't work (it won't if it shows red below the boat), try to buy another one and place it in the water.