Any antibacterial soap that doesn't contain alcohol works fine. Anything alcohol based can make the strap feel "gummy" and can damage the watch. I just shower while wearing any of my Garmin devices and rinse well and don't have any problems .
that manual statement is basically to prevent the tidepod generation from attempting to charge a wet VA3.
afterall the VA3 is 50m water resistant, meaning u can swim with it and by default also wash it. but yes, any sweat buildup over time can cause negative effects, hence still best to just run it under water and rinse well...then let it dry etc before plugging in.
I'm with Everdying, I personally wear my watch in the shower every time, especially after a sweaty workout. I lock the watch, then soap my arm making sure i get the watch under and over. Rinse it off then remove the watch. My watch smells as fresh as day one :-)
Everdying I've been using the same plugs on my 935 that the VA3 uses. 9 months of wearing it 24/7 that includes showering, working outdoors in the rain and both pool and open water swimming and I've never had water get into the charging port or any moisture accumulation.