I made this bridge in the scenario editor. Naturally, land units can walk across it, but water units can't go under it.
Also note that this is a fairly crude version of the bridge made primarily to show the most important features. If I were going to put something like this into a scenario, I'd have to spend a bit more time to clean it up and make it look better.
Here's a screenshot of the bridge:
ScreenshotSo, any comments or suggestions?