"It also forces the enemy to give some thought to defending stretches of river bank which would otherwise be completely impassable barriers. You could concentrate all defenses just on the bridges and fords. If assault boats are present, then you need to worry about the whole river. This is especially true given that the attacker gets to choose exactly where on the river to attack."
dont know if the a.i. thinks this way, human would though.
fredrocker's angel of sicily, you start out on the beach without boats. the boats come as reinforcements later for exiting. works well for me. but you only travel one tile.
i can see a scen where you want to delay the arrival of a force, and make them travel slowly. this gives the player options as opposed to a reinf flag.