for $5,000, or half of the estimated value, in exchange for a $100/month fee for leaving the boat there.
If that isn't acceptable then you have no choice but to sell the boat for the first reasonable offer, less $100/month off the top, and splitting the remaining proceeds.

But that’s just me.

Good luck.

He is your son, find out ballpark what the boat is worth, tell him you would like to keep it and see what he thinks. If he needs the money, sell it, if he wants the money, see if you can buy him out over time instead of taking the hit to your IRA

Maybe you have a neighbor or two who might like to buy part interest in a boat.

Make a list of all the money and time you have spent on the boat and convert your time spent to equivalent dollars.
Find out the value of the boat.
Deduct your costs from his half of the boat if you sold it.
Then pay him and keep the boat, or sell it. Your choice.

Personal: Stepson living thousands of miles away found out I had moved in with his mother. (Both our marriages failed at the same time and we teamed up. Doing ok 35 years later.)
Stepson offered to sell me his caravan, parked in the back yard of his mother's house.
Except it had rotted out and I had cut it up for scrap. Sorry kid!
(Didn't charge him for my labor or dumping fees though. Nice of me, right?)

for when that day comes.

I replied above but also

Have you asked him what he thinks/wants to do with his interest in it? Maybe he wants to retain his half or perhaps he knows someone who has expressed interest,,,, This isn't just your problem, it is also his.