This is a short (two or three minutes) scenario which demonstrates how you can create a bridge in-game which is then walkable and sailable for land units and boats respectively - it actually acts like a bridge, rather than like water or land.
I don't know if this trick (if you can call it that!) has been discovered/utilised before, but I haven't seen it anywhere so if it isn't new, ignore this and remember that I didn't mean to rip off anyone's work!
The trick does have some limitations; it is primarily an aesthetic tool for designers so precautions may need to be taken to avoid units getting in the way of trigger-created objects, and the area which will become a bridge may need to be cordoned off for part of the game (this is mainly for RPG's). Any comments, there is a thread in SD&M (http://aok.heavengames.com/cgi-bin/aokcgi/display.cgi?action=ct&f=4,40639,0,365) because I keep missing things if they get mentioned in the blacksmith directly!