What happens here? The marina has a vacant 50ft slip. The boatowner has a 42ft boat. Assume both lengths are real/actual. Does the boatowner get the slip, does he pay for 42ft or is he told "it`s a 50ft slip,want it, you pay for all of it".
I`m sure this gets determined by supply and demand. Not dissimilar to having a property for rent at $500pw with the market more like $420, I might eventually get $500, but I might drop weeks of 100% of the rent, whatever it really is, waiting for a taker. Maybe the 42ft boat gets moved to a 42ft slip if/when one comes free.
I don`t see the problem with telling the marina "it`s an XYZ 42". Marinas know boats, their business is berthing them,there`s no deception, just a fair description.