I sold a 2001 Quintrex 600 Offshore (wot a joke of a name for one of these) in 2004, 3.5 years old with 200 hrs on it. I got $1000 less than what I paid when it was new. The things I lost out on were a rocket launcher and GPS/sounder.
I've just sold my 2004 Fisher 660 Maxi. I had her for just over 5 years, and put 500 hours on her. Similar story. Lost out on sounder/GPS, plus storm covers but sold her for the exact amount I paid for her 5 years ago.
Depreciation??? Apart from the running and up-keep costs, my boating has been a relatively cheap affair
What has worked in my favour has been the ever increasing cost to buy a new boat, therefore the scond hand ones have held value. Down side is that each time I upgrade I have to fork out more. Not complaining though as if it was a car I'd be losing heaps in depreciation and paying more to upgrade too.