Once you have established the boat's actual waterline level, you can also use a laser level to place the waterline masking tape. It's easier at night to see the solid red line. You may have to block up the tripod legs to get the height just right, but then it's really easy to singlehandedly perform this trick. The boat has to be perfectly on its lines, though. I use a two-by spanning the cockpit
seats port and starboard, to confirm athwartship level, and the seat, itself for fore-and-aft level. In the yard that means playing with hydraulic jack and shims for a couple hours, but once it's done, and the hull scribed, you are set for life.