I know, I know, charging into a stream is great fun. Especially on a hot day. So does your bike, which is okay except that along with your bike getting soaked, the bike's bearings get soaked. That isn't too bad either if when you get home all bearings are greased immediately. Realistically, that is rare.
Riders go home, park their bikes, crack some beers, and relax.
Meanwhile the water goes to work. Soon bearings and races are corroded. Sand is as bad as water, maybe worse. If you doubt the possibility of sand penetrating well-greased bearings, ask the mechanics at Rim Cyclery in Moab, Utah. Imagine all those expensive steel bearings smoothly rolling around and around in their coating of grease.