Decided to necro since stealing a jet ski seems like it was supposed to be possible, but it isn't without triggering hostility. Which would be fine, BUT it's very glitchy and you don't nessecerily trigger hostility if you avoid coming back to that map. I'll try and explain all the points.
1. You're at Ray's shop.
You can break into his shop, steal stuff, and that'll trigger "Ray was stolen from" as normal.
If you break into the outside jet ski area that's also fine.
2. The jet ski camera
The jet ski camera is weird. It never turns, and it behaves more like an NPC with how you can't just walk behind it to break LoS, you need to walk outside of its sight range.
If you move to the bottom of where the jet skis are held you'll outrange it, and be able to steal a scooter.
If you stand just in front of it you can steal a Freighter without being seen.
If you get near Core City shore with that stolen ski you will turn everyone hostile.
If you murder a drifter, steal their ski, and return you will NOT turn anyone hostile.
3. You go to black sea and come back on a stolen ski [Edit: By stolen ski I really mean any ski, once you take a ski even unseen the die has been cast and the following will happen]
Upon returning from the Black Sea if you enter the map with Ray's shop everything will be fine for 30 seconds until some camera-hostility related code updates, and turns Core City hostile <<<this is the part that doesn't make any sense
If you return to Core City and don't go back to that map no hostility will ever be added
So things seem to mostly operate fine until you go to Black Sea, then come back. This is when you deal with this delayed hostility problem that only occurs when entering the Ray's shop map. It happens even if you never turn the camera facing the jet skis red.
With more waterways content being added, and skis having their price halved, can this bug be fixed in an upcoming version? Ray will even tell you how many jet skis have been stolen, though you can only talk to him in the 30 second period before hostility is updated. It feels like the whole thing is meant to be possible, and it has content related to it with the whole "Stealing from Ray" event, but there are just some strange hostility bugs likely all linked to the strange camera facing the Jet Skis and how it handles hostility. Maybe move the freighter indoors if you're afraid of it being taken, or just moving it to the center so you have to be in front of the camera to hotwire it.
tl;dr You can steal a ski, but you get retroactive hostility when you go to black sea, and only if you return to ray's shop, and ONLY after about 30 awkward seconds of no hostility.