That is a very good question.
Waterline length is the length of the boat from bow to stern where it sits in the water.
In other words, as it floats in the water if you were to mark the point on the bow where the water touched and marked the point on the stern where the water touched and then measured that distance this would be the waterline length. The boot stripe is the line painted on many boats which separates the bottom which is underwater from the sides that are above the water. Yes, the length of the water line does effect the speed of some boats. In particular displacement type boats. These are boats that have a large underwater profile such as sailboats and trawlers.