Rockdag, maybe you've never heard the famous quote about what a boat costs? "If you have to ask, you can't afford it."
I've never seen hard numbers for boat depreciation aside from what the IRS would allow as with any motor vehicle or asset in a business. If you want hard numbers, go to a large library, get the past 20 years worth of the BUC book (or buy the numbers from BUC) and you can actually measure the depreciation on boats over the past 20 years. Remember to find out the market interest rates for your loss of capitol and other factors in the same time. Remember that when Congress passed a 10% luxury tax on new boats...the prices of old ones soared as the market tightened up. And when oil prices rise, fiberglass resins rise, and used boat prices rise again.
The only other guide I've seen is if you buy the used boat review books from Practical Sailor. They show market values (new and resale) for the boats they cover, not adjusted complete depreciations. But they also show a lot of variation from model to model, and you'll have to accomodate that as well.
Depreciation? You're gonna have to do the homework. It would be nice if you shared it with us all after you had some numbers.