A few ideas come to mind:
Find owners that are renting
already. They're already accustomed to loaning out their vessel (however, for sleeping only). You'd need to talk them into letting you charter
it out. I'm sure they'd want references
From talking with the hosts of the AirBnb boat we stayed on a few weeks ago in Hilton Head
Island, SC, winter rates are drastically lower, and if they can't fill their dates, you might have a chance.
2) Local Marinas
Ideally, you'd find a boat that isn't being used for the winter by its owner. Contact individual marinas
with your proposal. They might know of some owners who are looking for extra money
, or at least may put your ad in the marina if you ask nicely.
Would you stay in the slip, or go cruising? I doubt the marina would want to help if you'll be removing the boat from its slip (since they'd be losing money).
3) Buy, use, then resell, as stated earlier.