What could I find really good bath tub floor adhesives? (details)?

My boyfriend is afraid of standing to take showers because his balance isn't all that great and he could easily slip and fall. He's already fell once and injured his leg. Now even thinking about falling gives him slight anxiety. We've tried a bath mat but it kept coming up and rolling up under our feet, and we've tried adhesives but none of them had a good non-skid grip while wet. Does anyone know of a brand of tub adhesives that have a really good grip and where I could find them? Thanks! :)