Lost a settler on water tile

[Svedin]

During my (first ever civ6) game I did not protect my settler well enough so it got captured by barbarians.
When I came to rescue the settler, I destroyed the barbarians with my regular melee troops and then freed the settler with a ship (Norwegian longboat). The settler then automatically moved out to the water square and was unable to move from that square! When I tried escorting the settler with the longboat both units were unable to move! Bah