Has your marina management actually told you that? Most marinas I'm familiar with just charge by the boat's length on deck. For instance a Catalina 22 would be charged for 22' even though it has an outboard hanging off the transom.
Per their printed rules:
Overall Length: The expression "overall length" means a measurement in a straight line parallel to the keel from the foremost part of the vessel to the after most part, including sheer, bowsprits, bow pulpits, rudders aft of the transom, outboard motors, dinghies, and associated mounting hardware.