They are supposed to be attackable under bridges. So are sea-creatures.
However, bridges should not stop ship movement. It might take 2 movement per bridge space, but it shouldn't stop them.
Bridges count as "open terrain". Found this out the nice way when my pikemen destroyed Piranha's stack of sea serpents. Could be justified -- under a bridge is hardly the open ocean. Don't know about pikemen on a boat under a bridge. Would they still get their open bonus (even if it's then cut down by the boat max?)
Jeremy, I meant also that unit that goes on the road and then enters a bridge cannot move further when he meets a ship placed under the bridge. If I want to go through a bridge why I am stopped with a unit placed under the bridge? I think such unit must have possibility to go further without need to beat ship first.
The game is 2-dimension so to say. So you can never move through an enemy unit. Otherwise it would then be OK for flyers to move over enemy stacks also I supose
As for the ship/bridge, I see it as the ship is putting up a blockade, and bombarding anyone who tries to pass, so there must be a battle Normally ships are pretty vulnerable on bridges though due to their battle-limit...