A reflector or mirror could bounce radio signals, but its size depends on the range between the bridges and probaly other things as well. Im not an radiowave expert but if the watertower has a cylindrical shape i bellieve the signals would most likely bounce back.
Another option is to put a 1310 on the tower in repeater mode. This would then repeate the signals from the Root bridge to the 1310 in Workgroup-bridge mode (a non-root bridge cannot connect to a repeater).
First off: you are correct in that true line of sight is really the best way to go (obvious: Duh).
If you don't have LOS, the sites are "close", and you have some pretty tight antennas ... and... you have an intermediate building (that does have LOS to each end site) ... you can probably do a bounce.
To qualify "close," using a bounce will cost you probably somewhere around 80-90% of your signal (depending on your "reflector"), distance is going to relate to the strength and quality of your signal and the transmission components.
The chances are probably less than 50% for success, so it's a pretty high risk for the expense and time.
As a side note: There are Amateur Radio operators that communicate with each other by bouncing signals off the moon. They get to use more power and usually have some pretty slick antenna rigs ... but given the distance and the rough reflective surface (of the moon) ... the fact that they get anything is encouraging. 2.4 gig is a much "bouncier" wavelength then the 2 Meter, 70CM, and other freqs used by the moon bouncers.
I wouldn't count on a water tower as a reflector, the convex surface would disperse the signal, a good flat surface (like a building in a downtown environment) would be your best bet.
As mentioned by the other poster, a repeater station is a much better bet, if you can get access to a point ~midway (and if it has LOS to the other endpoints).