Just a few that I can think of off the top of my head:

1-Mfg suggested retail when new
2-Length (point of prow to transom)
3-Material the boat is constructed from (hull and decks)
4-type of powerplant and drive (inboard, outboard, stern drive, jet drive or propeller, gasoline or diesel)
5- who manufactured the boat (some brands naturally have a higher quality and value)
6-who manufactured the engine (same as above)
7-custom features
8- probably lots more.

All these come into play to establish a value. The age of the boat and how well it's been maintained, and whether it has had storm or collision damage also plays a big part.

What kind of boat are you thinking of? Are we talking runabout/ski boat, sport fisher, yacht, dhow, sailboat......?