Boats drop in value quickly because of maintenance. Sea can be a tough environment, sea water is really corrosive, you need to regularly anti-foul the hull, specialist gear is not cheap etc. You can keep the costs down if you have the time to learn and do some of the work yourself. Wooden boats are even more difficult to keep in good shape. Moreover, you need to rent a mooring for it, which can get really expensive depending on location.

That's the main reason why people half-jokingly say that the two happiest days for a boat owner are the day when they buy it and the day when they manage to sell it. Still it's worth all the effort if you have the time to sail it regularly. Otherwise, chartering (renting) a yacht for a period of time can be a much better option.