Water retention is probably more a matter of being fully fueled on glycogen storage than most other things. On a long, brisk run on a cool/cold day where you are slightly under-dressed you will find that you have dropped a lot of weight because the respiration will dump a lot of water that is picked up in the vascular system.
You can go into a race without adequately replenishing these supplies but will run out of fuel sooner. If your race is not up hill, I would not think that weight matters that much, especially since it is not real cell weight that has to be kept metabolically active (e.g., excess fat or even excess muscles).
You can experiment but I would not do so in any sort of race that was important and I would drop out if anything starts to go wrong. I would be especially worried if the race is in a warm/hot environment. Wrestlers do all sorts of stupid things to make weight that have no place in an athletic environment and only related to qualifying for an easier group.