There is a rule
of thumb, not particularly accurate, that it costs 20% of purchase price to own a boat per year. That includes maintenance, moorings, insurance
and the like.
That does not take into account depreciation or capital amortisation or the like, which is hard to generalise about, as one boat will depreciate, while another might not. For example, boat "x" may have cost $100,000 in 1980, but will still sell for the same price $100,000 today. Theoretically, no depreciation therefore. However, in real terms, the vessel has depreciated, as inflation has eaten into the $100,000 figure.
If you have some old boat magazines, it's interesting to look at the prices of the same boats a few years ago. That will give you at least an idea of how to work out your depreciation (if any).
Some boats actually appreciate in value, look at some of the classics for example, but again, in the real world, the cost of maintaining them would totally offset that appreciation.
One thing you can be sure of, however, is that boats are not as bad as cars. And how often do people bother to calculate their depreciation on cars? Truth is, its rare, as far as I am aware, for people to worry about their depreciation on boats.