I think it's safe to say there is an immediate depreciation on the purchase of a new boat - much like in automobiles. The 20% figure could well be correct. But beyond that there are too many variables to come up with an accurate generalized depreciation schedule.
The reputation of the manufacturer, the condition of the vessel, and to a small extent the amount of extra equipment installed will have a huge influence on the market value of any particular vessel. Even regionally some boats command higher prices than elsewhere.
Many boats have been sold years old for more than they cost initially due to the effects of inflation and local market forces.
Selling prices are one thing, ROI another altogether. As mentioned above, we all throw plenty of money into our boats over the years and it's certainly not realistic to expect much of that back.
We were fortunate, in our view, to have owned a couple of different boats for more than a decade, and later sold the boats for nearly identical prices to what we paid. Did we get our money back? No way, not when you factor the improvement and maintenance costs along with operating costs. Do we feel we got our money's worth? You bet we did.