How do you paint a waterline on a sailboat?

When the boat is ashore, on many you will see a groove where the water line should be.The groove maybe at the boot top and not at the waterline. You need to know your design freeboard. Make sure your boat is level,on level ground, make a tripod with a chalk, that you can run around the boat. Mark the waterline at the lowest part of the sheer.

Another way is leave it in calm water until the slime builds up and use the slime line.

Well, your question seems to be asking how you actually physically paint it, and the answer is that you paint it while the boat is out of the water...just at the top of the antifoul.

If you're actually asking how you know *where* to paint it... it's usually marked by the builder - BUT don't forget that loading or buoyancy may have changed, and the waterline could have moved. In which case, it's best to mark it yourself, before taking the boat out for painting

There is a procedure to strike a water line evenly on the hull when the boat is dead level in a cradle. Too lengthy for here. It Takes some appropriate sized saw horses, string and tape. That is if you have "lost" the original water line. If the whole boat has been sanded down severely & line is gone totally you need to figure out few waterline stations while boat is floating. Then re-create a dead level line on the dead level boat.Painter's tape is used by conforming of tape by eye to string.

With a paint brush.