Many, if not most, production boats, have a score in the gelcoat (from the mold) that shows the designed waterline. It is easily found on new boats, but sometimes impossible on used boats. You can easily tell the real waterline on a boat that has been in the water if it reveals a water stain. That can be your gauge. Waterline stripes are cosmetic. You can be as creative as you wish and can afford. Some boats have triple stripes of different colors even. Some a thin single strip and some a much thicker one. Some have a thick one and a thin one. It is up to you, but depending on the hull shape, shear, etc. a stripe can really make a boat look a lot better. Boats that dry sail/race often have a completely white hull and the owners like that look. Not so frequently boats that need antifouling will apply the antifouling without a waterline stripe. This, in my opinion, looks better on a one design boat, but it is all up to you. The problem with vinyl tape is that the tape is the same dimension from start to finish and if you look at painted on (or in the mold - gelcoat) stripes, you will see that the thickness varies with the turn of the hull at the waterline stripe. The bow will be thicker, midships will be thinner and at the transom the thickness can be 3 or 4 or more times thicker depending on the run of the hull and the size of the transom and where it sits in relation to the waterline at the stern. Vinyl stripes are easy to apply, while paint is more difficult. But, like anything, more work usually produced a nicer look. If you are going to use paint, then I would stay away from polyurethanes if the stripe will be submerged as it will blister. Paints like Awlgrip are great for above the waterline where is is temporarily wet, but not constantly submerged. If under water the entire time, it (along with Imron and other polyurethanes) will blister. So, while it is used above the waterline all the time, it is applied above the waterline. The antifouling can meet it, or you can put a white (or whatever color your hull is) space between the waterline stripe and the antifouling. It is all in the cosmetics. Interlux makes waterline paint (called bootstripe) in basic colors (navy blue, black, white and red). It is good because they sell it in small cans and you don't have to buy a quart of paint to do a small stripe on a small boat, for example. You can buy a laser beam device to use as a guide to make sure the waterline top is level (the boat has to be level too) when the boat is on the hard. Or, with some help, you can stand back with a step ladder with a 2x4 on the top, leveled off and eyeball it at the level boat and guide someone to make a pencil mark at the right spot along the hull defining a straight line. The straight line will not be straight on the hull, but will look straight from a distance even though the hull is curved at different places. A harder way of determining a straight waterline is to use a clear hose filled with water. Supposedly water always finds its own level, so keep the ends of the hose above the water line of the boat and the water level at each end should be the same and so then you can determine the this position anywhere on the hull. This sounds easier than it really is, however.
