Not sure if this is a bug or a quirk of how terrain bonus's work.
But when I park a unit with an open bonus under a bridge (as in the unit is a ship) it gets it's terrain bonus. This makes Pikemen the ultimate bridge blockers of boats/sea serpents because they are getting +15 bonus while on the ship under the bridge.
Personally I think this is a bug in how that square is handled but others may see it as a feature allowing Pikemen/Lt Cavalry/Hv Cavalry/Elephants in boat form to find a safe spot while traveling in boats.
KGB