While having previous mod I had boats stacked on each other with a sim swimming inside of them.
I downloaded the 44c version and play tested for 1 sim week till now. I have 1 boats abandoned by the shore and one other boat with a sim swimming inside. There is also a sim that is standing on the little island not far from the main land (Monte Vista) and when I first saw him he was in his swim clothes, so I assume he swam there, but he wasn't moving and there is icon of not being able to move, walking and boat alike.
Overall I see less sim with boats stuck in the water, but it's still there. Furthermore when I check object stats I have boats far in my unroutable terrain.

This one is pretty difficult to nail because the boat routing doesn't seem to respond to routing flags like the cars do and it's deeply woven into the posture code. The best test cases I can manage are to tell the Sim to go somewhere on the water and see the result. I haven't found them doing anything other than swimming when I instruct them but clearly there's something I am missing. Figuring out what is the million dollar prize. If you can catch them in the act and see what interaction is spawning the boat that would help a lot.