Observed on v1.5.0 (Tempest).

The Ardent Mages questline has an early stage where you gain two units by visiting a ruin. If that ruin only has one adjacent land tile, then the new units end up in the water (since the only adjacent tile is occupied by your visiting army). The new units are then unable to move, presumably because Shipyard hasn't been unlocked yet.

Screenshot:

In the situation as shown above, I'm unable to move the army in the water - right-clicking does nothing. I can't land on the ruin, and it's presumably not letting me move on water because I don't have the required tech.

Save file:

Stranded.zip

(I'm using a custom faction, but I can't imagine that's significant in terms of the bug)