Okay, I could do this, following an appropriately-sized major crime lottery win:

First improvement I’d make: a clay pigeon station (on the ocean side, away from prying eyes behind binoculars). Second: hang some steel plates between the dock pilings, for some .22 plinking.

Third: some kind of seagoing vessel (screw that helicopter foolishness), for both commuting and fishing purposes:

…and lastly, a Barrett M82A1 rifle, to repel any prospective intruders from distance:

I have the shorter-range issue well covered, as you may imagine.
Feel free to add your suggestions in Comments.
Looks decent on the google maps
https://www.google.com/maps/place/Marathon,+FL+33050/@24.6871845,-81.074598,158m/data=!3m1!1e3!4m6!3m5!1s0x88d0dc33cb6c387b:0xbcb6224194d144f7!8m2!3d24.726026!4d-81.0446262!16zL20vMHJwNTA?entry=ttu&g_ep=EgoyMDI2MDkwMi4wIKXMDSoASAFQAw%3D%3D
First thought, that place would be nice until the next hurricane, heck the next ambitious tropical storm came along… better reinforce that seawall and have a better entrance. Second thought, as a RSO, I’m thinking you ‘what’s beyond my target’ analysis is a bit weak. How far away are those boats from your proposed .22 plinking range not to mention passing vessels. Love the idea, wish you were a bit further distance from ‘civilization’. And in the spirite of Random Totties, how about a ‘Random Totty Cottage’ on the premises for visiting ‘photo ops’?
The listing says $ 16 Million so a decent sized Covid Loan Fraud would have been enough instead of a lottery win. And Massachusetts is just now uncovering a number of them now that it’s election season and the fraudsters are not following through on their promised contributions.
The listing claims a helipad, but the photos show a H more suitable for a drone, not a full sized Helo, Plus no public utilities so water and sewage is a problem that wind and solar power can’t fix. Plus it’s just one good hurricane away not existing. Try getting insurance for that property.
Hard Pass
Given the luxury of the place, I can’t believe the house doesn’t have fresh water and sewerage taken care of. You wouldn’t be able to rent out the place otherwise.
Also, given that the place has been around for over a dozen years, it seems to have survived all the tropical storms and such quite nicely.
My Fathers place on the Gulf ( south of Tampa right on the beach on the gulf side of Casey Key) survived nicely for 30 Years until a direct hit by a Cat 4 storm destroyed it 6 years ago. How far are you willing to push your luck? Gulf side Hurricanes are unusual but not unknown. The Keys are a completely different story in that storms that don’t turn right to follow the Gulf stream head straight across toward the Keys toward Mexico, South Texas or hook toward Nor’leans
Sewage is likely dumped directly into the ocean ( feeding all those crustations and other fish) , and Fresh water falls from the sky, and like many island properties collected into large cisterns somewhere on the property, usually enough but sometimes not.
Mr. du Toit:
I went backwards through your link to the actual real estate listing. They note that the place is completely off-grid and has its own wind and solar power systems, along with a emergency backup generator. The sewage treatment is on-site, and they have a dedicated desalination plant along with a cistern.
Here in NW Wyoming it’s not unusual for a remotely located house to not have a well. In lots of locations there’s simply not an aquifer within 500 feet of the surface, and sometimes there’s water but it’s so alkali that it can’t be made potable. These folks typically purchase their water at a local treatment plant. Some towns have a spigot dedicated to this, with a credit-card metering system for payment. They put it into a tank on a trailer, and haul it up the mountain (or out into the desert) and fill up their cistern. It’s then pumped into their fresh-water system. I’d guess that the system at this mini-mansion is about the same.
But I’d never buy something so incredibly vulnerable to just one big storm. Be fun to rent for a while, which is what the current owners do. It’s something like $12,000 a week, which goes up to $17,000 on holidays. I’ll let you know when I win the lottery and invite you over.
Needs to be further above the waterline (the inevitable hurricane) and further from the coast – preferably outside the 12 mile limit . . . .
then we can talk.
Looks like a nice enough place. Has solar and wind power and, hopefully, a decent size battery bank so electricity is covered. I cannot imagine it doesn’t have running water but a home sized (100-300 GPD) desalinization system can be had for about $150K or so. Be sure the electrical is up to it. I’d be worried about storms but, if you can affords that place, you can afford to evacuate for the duration. Heck, maybe own or lease a pied-a-terre just for the forays to the mainland.
Better be sure new wife is OK with it. It is not isolated but women are different with regard to socialization.
One nit. It is on ocean side so you will not see sunset over the ocean unless the Earth’s rotation changes for a mere $16 million.