I’ve never had a problem with gripping when using hot water with my jelqing and stretching, but then again, I don’t take baths, I take showers, so I just aim the spray at the base of my shaft. Splash droplets usually get the rest of my shaft.
As for stretching/jelqing when your penis is actually submerged in water (i.e. in a bath), maybe wrapping something around your penis before bathing might help to give you something to grip on to? I don’t know, since I’ve never tried nor had to try it, but it could just work.