It's a curly one, depending on who is doing the measurement is what determines how long it really is eg. A marina owner may well measure to total length from whatever is at the front to whatever is last at the back (swim platform?).

An insurance company may measure you at the max length to grab a few extra dollars for your premium, eg. a boat may actually measure at 28' but the company calls it an XYZ 30.

The US Coast Guard defines it like this "Overall Length - The horizontal distance of the hull between the foremost part of the stem and the
aftermost part of the stern, excluding fittings and attachments." This one I think is the correct and fair way of measuring.

Usually from the most forward part of the permanent hull structure to the most aft permanent part of the hull, sometimes referred to as LOD (length on deck).

LooseChange said..
Usually from the most forward part of the permanent hull structure to the most aft permanent part of the hull, sometimes referred to as LOD (length on deck).

This is how it's done for survey purposes and for fishing vessel licensing. Amateur boats the figures are often rounded out, shorter or longer and in metric or imperial to make the name! Who wants to look at an SS33.5.