[Ed]

Definitely strike the waterline with a grease pencil first.....nothing looks worse on a boat than waterline that is way too high.

Another hint for when you mask off the waterline. Affix the bitter end of the tape to either end of the boat....and then walk down the length of the boat while unrolling the tape. Once you're at the other end of the boat and have sufficient tape unrolled, cut/rip the tape, pull it taught, make sure the waterline is level and then rub the entire length onto the hull.The edge of the tape acts as a straight edge and assures a straight line)

Too many people make the mistake of unrolling & applying about 3 feet of masking tape at a time. Usually, it results in a waterline that is wavy and not level.

Good luck.

Ed