The water pistol full of piss made me laugh. I haven't used one for a while, but I don't EVER remember having one that didn't leak, so you might need to wear a waterproof until you've used it. Presumably you'll tell him he's about to get a faceful of your piss if he doesn't back off...
...hmmm, not sure how that would go down with a bully, in a pub, in front of his mates...
Perhaps you could get a supersoaker, thus benefiting from a more powerful jet and a longer range, thus enabling you to get him from the other sid of the bar. You might have to get YOUR mates to help fill it though.