I sweat a lot when on the elliptical trainer, but so far my main problem has just been how gross it is to see all that sweat collect under the band. I'm not sure anything actually gets inside the mic or speaker cutouts. If it does, it's very minimal. I just give it a quick wipe down with my damp towel after I shower--seems to clean it off just fine.
For Apple Watch, I just treat all the torture tests and water tests as proof that this thing will hold up to every day real life, for example being caught out in a rain shower. I have no illusion that it was designed for swimming other kinds of regular excessive water exposure. I've seen a lot of my watches get water damage just out of the blue, not even knowing or remembering what I could have done to cause it, so I'm still cautious about water and watches, even when they're highly rated for resistance.