Ug, the term is on the tip of my tongue! What is the nautical term for the markings on the side of a ship to measure the waterline? I want to say they are called draft marks? Dang, I know it’s a short word like that, IIRC… - Jinx 
It sounds as if what you’re looking for is Plimsoll line.
Draught marks. Actual short lines may be used, or merely numbers, in which case the draught is read from the bottom of the number printed. Typically, draught marks are painted fore, aft and amidships, so that list and pitch can be observed. In addition to the draught marks, there may also be load lines, also known as Plimsoll lines, which indicate maximum loading conditions of the vessel with seasonal or sea state variance. Other waterline marks which may be present are the load line certification issuer, bulbous bow indication, and bow thruster indicators.
draught or draft marks. On the boat I served on, the numbers were 6 inches tall with a six inch space between each number. 23’ 4" forward and 33’ 9" aft. And before I hear “It’s a ship not a boat!” I served on a submarine and ALL submarines are boats. Just ask a submariner and he will tell you so. 