Normally the length of the boat is given by the boat builder and is part of the model number such as Beneteau Oceanis 373 for a 37 ft model.
The main two important lengths on a Yacht are Length overall (LOA) and Water line length or (LWL) Load water Line
Length over all (LOA) is the measurement of your vessel's length including any spars that protrude from the hull or platforms on the stern. Most boat builders quote for their boat sizes.The importance of LOA is obvious when docking in a marina slip, especially when you need to account for a dock wall or bulkhead. You don't want to lose your bowsprit or damage the marina's facilities by not knowing your proper LOA.
Waterline Length or Load water Line (LWL) This is the length of the vessels hull, from center fore to center aft at the level of the water. This is the primary factor in determining hull speed. There may be a large variations between the vessels overall length LOA and the waterline length due to overhang i.e. the LWL may well be quite a bit less than the LOA. For example older sail boats had traditionally larger overhangs at the bow.
Length On Deck (LOD)
Technically, the deck is often (though not always) shorter than the entire boat, but it’s shorthand to avoid confusion and usually means the boat without the spar overhang.
Length Between Perpendiculars (LPP or LBP) The distance measured along the summer load waterplane between the aft and fore perpendicular. The aft perpendicular is taken at the aft side of the rudder post and the fore perpendicular ‘is the vertical line that intersects the forward side of the stem with the summer load waterline.
Hull Length (LH or LOH) The distance measured from end to end over the deck, excluding sheer. It is the straight line measurement of the overall length, stated in feet and inches, from the foremost part of the watercraft to the aftermost part measured parallel to the centerline, excluding outboard motors, brackets, bowsprits, rudders and similar attachments.