[MRM]

As pointed out above there are different measurements for different purposes. The marina you dock the boat at will probably charge you for total overall length. Sometimes the bowspirit or gangplank is not included, just the extension that is structurally part of the boat. Kind of like with buildings the height measurements include spires (which are part of the structural design of the building) but does not include antenas (which are just added onto the structure). Length at water line is probably the most common measurement and the one you'll use to register the boat. Widest width is called the beam.

So if the boat was 50 feet long at the water line and 16 feet wide, you'd call it a 50 foot boat with a 16 foot beam and people would understand what you were talking about. .