Have a ship in the open sea away from the shore. Tell a unit to garrison in the ship. The ship should move to the shore so the unit can garrison. (It previously did this; just make sure it still works).
Directly tell the ship to move to a specific point on the coast that the unit can reach. After the ship arrives, tell a unit to garrison in the ship. The ship should not move, and should just wait for the unit.
Tell the ship to move to an area of coast (a cliff, or another island) that the unit can't reach. Tell a unit to garrison in the ship. The ship should move towards the unit.
Have two ships offshore. Tell the unit to garrison in one ship, but before it finishes doing so, tell the unit to garrison in the other ship instead. The second ship should start moving to pick up the unit.
Performance test: tell many units at once to garrison in a ship, on a large map that's mostly land so that the number of reachable hierarchical pathfinding regions is large.