A building known as Stack Rock Fort is on the market, and that name's not overselling it. It's a proper fort, from the 1850s, built on a rock in the sea that the seller is generously describing as an island. It's a small, rocky island, with a substantial amount of sea view.
The asking price starts at around £400,000, which isn't much compared with a two-bed city pad, but is quite a lot for a derelict grade II listed house on an island that's not much more than a dungeon at the moment, and one that's going to need probably millions spending on it; if you can find a builder who's also a boat owner and not afraid of mouldy things and can work out how to get scaffolding up above the sea.
As for whether you might actually want to buy it and turn it into an exclusive AirBnB escape hole or offshore celebrity sex dungeon, though, that all comes down to the internet speed. Rightmove's connection speed checker says up to 76Mb ought to be possible, but that seems to be using a postcode on the mainland as a reference. There's no fun in having an impenetrable island fortress if you have no internet and therefore nothing to do out there. [Rightmove via Wales Online]