For the first time I was able to get far enough into a game to build a frigate. I moved a pioneer on board it, and moved down the shoreline a bit:

Note that the frigate is selected and shows the pioneer on board. When I select the pioneer and try to move him ashore, he shows up (sort of) back at the point where he boarded the frigate, and cannot be moved:

I have a saved game that shows this here.