I marked my waterline a couple of weeks ago with a $32 laser level I bought from Amazon for this purpose:
https://www.amazon.com/dp/B07JX7NG7X (there are dozens of models available in this price range). It worked great. I used the laser for leveling the keel as well as actually marking the water line. I was happy that the marking could be done even if the laser was only 5 or so feet away from the boat, because my cramped working area wouldn't allow me to get it further away.
My goal is to duplicate the waterline and boot stripe layout of PS #1, since it looks nicer than any other PS I have seen photos of. The manual says its waterline was marked 16 1/2" below the bottom of the keel, but I found that I had to set the laser an inch or so lower to match the final appearance of PS #1. When I actually paint the white boot stripe, I'll use the laser again to mark its location.
Doug