UPDATE: OPERATION GRAND THEFT AQUA WAS A TOTAL SUCCESS
Yes, it is possible to steal a jet from Ray even before expedition starts. I will post the necessary steps here:
What you need: High perception to see the hidden vent (attempting to lock the outside door will get you spotted by the camera) and 110 effective lockpicking skill and one hotwiring tool
1 - Get to the hidden vent so you gain access to Ray's Shop
2 - Get out through the door near the watergate
3 - Stay directly BELOW the camera overseeing the Freighter, you won't be detected
4 - Start combat mode and make a run for the end of the pier by the Skimmer
5 - Exit combat and enter stealth (otherwise you'll get shot by that cunt Briggs)
6 - Pick the ignition from the Skimmer
7 - Enter combat mode, hop on the skimmer, pop a adrenaline shot and sail to the left
8 - DO NOT APPROACH ANY ONE IN CORE CITY IN THIS JET AT ALL, THEY WILL KNOW YOU STOLE IT AND WILL TURN HOSTILE!
9 - Sail to the waterways and look for Jet Skiers
10 - Murder some poor sap and acquire his ride
11 - Since you're not on Ray's stolen ride, you'll be able to go back to Core City
12 - Go to the Black Sea to trigger Ray's Shop
13 - Return to Core City Docks
14 - If you saw the thief on the vents, give his description so you get discounts for Ray
15 - Laugh at the expense of this dumbass who owns the monopoly of jet skis and has no clue you just stole the shit out of his shop
16 - Enjoy jetting at a relative low level, gaining access to the Rig and Drag n' Drop
Pic Related, Dumb Ray butthurt about the stolen jet. Also, note that he says "1" jet and not "one". I think it may be possible to steal more jets from him, but i don't see how.
Also, DO NOT DO THIS if you don't want to be a pirate. Aegis will shoot you if you go near their camp.