I had the same problem post Dec 19 patch but I have a [small/regular size] "ship" base and was unable to place anything on it. I tried the small/regular mast, helm, aft, and fore. The fore really pushes the ship down!
The boat will float again eventually or I can change the position of the anchor to make it float immediately.
Ship objects were always failing with red placement blocks no matter where or how I tried to place. It was giving me "not enough room" errors while the ship was above water.
First person or third didn't make any difference - same results.
The thing that ended up working for me was to place a single log on the ship. Then I removed it (not sure that this part mattered) and everything else was suddenly place-able.
Screenshots of the issue are in steam profile named "PeculiarPickle"
output_log.txt