Any time I sweat in my watch, I remove the band, take it into the shower with me, and wash it with the shampoo lather off my hands. I then just put on new band while that one dries. I got a 3 pack of knock off trail loops for $12 and this system has been working great.
I just use $4 knock off trail loops and wash them in the shower after every workout. I have 5 of them, so I just switch to a new one while the other one dries.