Maybe theyre trying to avoid zombies by swimming? 
they be fishing in da river!
nah seriously i have no idea what those npcs are doing there lol
If that’s the same bug I saw back in 02Feb, Dynamic NPCs (as in, not the one that was in your shelter at game start) tend to get into all sorts of places. I’ve had my map show them in solid rock, so swimming seems both pretty tame and overpowered at the same time.
(One Rock’d-NPC could be justified by Lab goodies gone wrong. Fully loaded NPCs swimming had better be strong & Fish-category mutants. (Tail Fin is pretty powerful.)