They don't really lower the level of the lake, it goes down in the fall when the boats are removed due to the reduction weight & displacement exerted on the surface of the water as follows:
Boat
The mass of the boat of M.
Water
The density of water is ρw.
Archimedes' principle tells us that the volume of water displaced is:
Vdisp=M/ρw
i.e. more water is displaced when the lake is full of boats, so the water level falls when you remove the boats at the end of the season.
So you see the lake lowering is a myth perpetrated on novice boaters to get them out of the lake earlier so that the rest of us can enjoy a few quiet weeks at the end of the season. Sorry to let the cat out of the bag,,,
Sorry I also barely passed calculus so best you don't rely on me for anything math,,,
