Hmmm...that's tough. You can't use events to make unpassable terrain passable. (I don't care if those are words or not...) What I'd do is just copy that map and make one exactly like it, only replace the water (where you need to) with a bridge. When the player hits the switch, have the player teleport to the new map (the one with the bridge), without a transition. It will simply look like a bridge has suddenly appeared, and you can now walk across it.
(For teleporting into the new map, you just need to use a switch operation that has your player teleport to the new map instead of the old one.)
It sounds complicated, but it's really just a matter of copying, pasting and teleporting.