Chances are, if the wheels are spinning fast enough to balloon, no water would be able to get in through the open slits because of centripedal force.

Do the slits add to the problem of water getting in your tires? Maybe, but i don't think so. If i squeeze the tire and let it go, i see no evidence of it sucking air in through the slits i've made. The vent holes in the wheels allow that. Water would be harder to suck in than air.

The fact remains that water is getting in with or without the slits. The slits help get the water out.