I might put in Jaws as the victor. Since they're fighting in water, Jaws is more adapt at swimming and attacking with its large jaws. Xenomorph is fast, but I don't think it'd be able to do a huge amount of damage to a shark that survived a gasoline explosion in the face.
There's the chance that even if Jaws seriously injures the Xenomorph, the acidic blood would injure it. But since they're fighting in water, the water would dilute the acid, so it may not do major damage to Jaws. And this is where the Xenomorph may start to die out.
Since it's blood is like acid, and it's underwater, the water would trickle in the Xenomorph's wounds, rapidly weakening the rest of the acidic blood in the Xenomorph's body, rendering it harmless and possibly killing the Xenomorph in the process through some chemical reaction. Combine that with its attempt to kill/fight off Jaws, and Xenomorph may not live.
This is just a wild idea in my head, but all acid dilute through water, so I can't see Xenomorph survive this.
The only way I can see Xenomorph win is if it chances a blow into Jaws's brain with its secondary maw; that thing's powerful enough to shatter skull. And even that may be very difficult to aim while fighting under water, and that's if it knows anything about a Shark in combat.
I can really see Jaws kill the Xenomorph and swim away with this with a couple of scars.