The term “waterline” refers to the line where the hull of a ship meets the water surface. The special marking of the waterline is called the National Load Line, or (my favorite) the Plimsoll Line. The purpose of the waterline is to ensure that a ship has sufficient freeboard (i.e. the height from the waterline to the main deck) and thus sufficient reserve buoyancy.
In other words, if you load too much aboard, your Plimsoll is no longer visible, you may loose your buoyancy, and sink.
Which is a bad thing.
With three furry creatures and two less furry ones our Plimsoll Line is still visible, but very close to becoming a wet Plimsoll Line. From our stance just barely above the water come the present Waterline Notes.
Thanks for reading this explanation! I hope it helped.