Roland Johansen
Deity
I seem to recall building highways over boats in Civ2, but think units stopped at boats anyway. I forget why I bothered to do this, unless perhaps it avoided the second stop on the other side and allowed continuous movement.
Might be a cool mod in Civ IV (without the permanent boat) -- think Confederation Bridge to Newfoundland. Maybe turn a workboat into a 1-segment road?
Note that tiles in the game would typically represent an area of 100*100 kilometers or more (if the planet is compareable to earth). It's not that realistic to create bridges of a few hundred kilometers crossing deep ocean tiles. The rivers visible on the map could already be representing significant bodies of water.
It might be interesting for gameplay if the rules would be similar to the fort-canals and if these bridges could be destroyed fairly easily.