If you are on two skis, try doing a sitting dock start. No drag through the water and thus no grip issue
To answer your initial question, the single best exercise for water skiing, including grip, is dead lifting. If all you have are dumbbells, work up to 60 # bent over rows. That will strengthen your grip and your core.
if all of that is further than you want to go, search “grip strength trainer”on Amazon get one, throw it in the car and use it while you drive to and from work