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. Now imagine sand in the grease and think about those balls rolling around and around. Save your bearings by either riding slowly across streams, puddles, and sand or carry your bike over them. But if you can't resist the trill, lubricate your bearings frequently, as in after every ride in which they are subjected to sand or water.