I don't know how big this race is, but if spectators are going to be there, then chances are some of them are going to be near the water crossing. If there are any spectators in the water, point your bike directly at them and cross there, because, they are more likely to find a shallow part to stand in than you. Just pay attention to how far the water comes up on their bodies.
Also, Try look for places with ripples in them, These are usually the shallower parts, the ripples are caused by rocks/obstructions under the surface. This isn't always be a good technique, since there are usually a lot of jagged rocks that could cause a flat. (try switching to foam tubes, then you can cross anything without having to worry about flats).