casioqv, you're forgetting the slip itself is an asset.
I realize that, but the association fee is a total loss, and it's more than half the price of renting.
With a regular house or condo the prices are such that a normal monthly rent amount will fully purchase the house in 15-30 years, while the buyer is still potentially alive.
With these dock-ominium slips priced so high relative to renting, you can never come out ahead until you sell it- and then only if you're lucky and it's retained it's value. If the place has become less popular/mis-managed/poorly maintained/etc. it could potentially lose all it's value.
It's hard to imagine any scenario where you wouldn't have greater profit potential and less risk renting the slip, and choosing a better investment for the money you would have spent to purchase one.