Greg Koenig has been running some (unscientific) tests on Apple’s Leather Loop band:

So far, this chunk of strap has been through 4 different soak cycles. Three of those cycles were a 10 minute soak, followed by air drying at room temp. The latest cycle was a full 8 hour overnight soak. Drying takes about 3 hours.

The result? The leather shows zero visible signs of damage, zero texture change. As far as these (again, unscientific) tests are showing, the leather on Apple’s Loop band is essentially waterproof.

(Make sure to read his disclaimer.)

Anecdotally, my Leather Loop band has been through showers, sea water, rain, and sweat. I took basic care of it, and it’s exactly like the day I bought it. One of my favorite Watch bands.