"The waterline" is a short convenient term for "the design waterline", i.e. the horizontal section through the hull
at which the designer
intends the boat to float. Normally he will state the displacement
of the boat when it is "half laden" i.e. when tanks are half full of fuel
and water, and half the anticipated weight of crew and people is aboard.
Few boats float on the design water line when they are first launched and the designer
expects the crew to fill the tanks and to stow their clobber in such places in the boat that she will float with the design waterline parallel to the water's surface either a little above it or a little below it. I.e. the boat may ride higher or lower than the design waterline - "the waterline".
At the yard, when the boat is launched and afloat and laden, the hull(s) is/are marked fore and aft where the surface of the water touches her. That determines where to put the upper edge of the antifouling, and therefore where to put the decorative stripe called "the boot top". In days of yore the hull
was marked for future guidance by inserting a round headed screw both fore and aft to mark that height on the hull.
If some amatoor along the way has "improved" on this, you cannot rely on what you call the waterline - the top of the antifouling - to be where it ought to be.
However, the fix is simple! The designer has, as well as having determined in his design how the boat will float, i.e. where the design waterline should be, also made sure that the deck
"soles" (what landlubbers refer to as "floors") are throughout the boat parallel to "the waterline". Therefore, if a spirit level placed on the sole indicates that the sole is horizontal in the fore and aft direction the boat is floating correctly, although not necessarily "to her marks".
If the spirit level indicates that the sole is sloping, then the boat is out of trim, but you can trim it by plcing weights fore or aft until the level sez that the sole is now horizontal. Then the boat will be in trim.
At this point the boat may float either high or low. There is a design parameter called "pounds per inch immersion". It tells you how many pounds it will take to make the boat "sink into the water another inch". Supposing that number is 1,800 lbs. If you are "two inches low", then remove 3,600 lbs of clobber and the boat will "float to her lines". If you are "two inches high" then you can afford to bring another 3,600 lbs aboard and still be "floating on your lines"
So there are two questions you have to answer to determine whether you are "on your lines": 1) is your deck
sole horizontal? and 2) are you high or low in the water?
If you check for fore'n'aft horizontality of your deck sole and then come back to us, we can be more helpful in determining whether you are "floating high" or "floating low"