It is very difficult to be definitive on this because the effects of exchange rate are often larger than depreciation.
I have analysed significant numbers of both vehicles and boats and
(a) assuming either is a production model rather than a special
(b) adjusting for exchange rate factors
(c) allowing for inflation
(c) assuming any special additions are not a significant amount
in the first year
Depreciation for motor
vehicles is 12%
Depreciation for boats is 10%
However, from this point on boats depreciate quicker than motor
I emphasise this is an average over many examples. In these threads people often post that they sell for more than they buy. This is most often attributable to exchange rate and inflation factors.