You can't shoot someone for stealing property. That's illegal and isn't known as a "justifiable homicide" in "self defense". It's if your life is in "great bodily harm or possible death".
Also, you can have a gun on you and are going to great lengths to do so. Why not also have your wallet on you? I don't get it. Don't get me wrong. In Miami, FL city I was born and raise I always have my gun on me. Rain or shine. Beach or hotel. If that's on me so is my wallet, keys, knife, ID's, phone, spare mag or mags. No reason to separate them unless I'm sleeping or in the shower. (The gun comes with me alone and either sits on the night table or toilet) yeah, home invasions suck.
Anyways, just keep it all together. No need to separate anything or justify why you want to carry at the beach. Obviously you have seen or dealt with something unpleasant. Just do it then. Also remember you CANNOT shoot anyone over property unless they are in your home or you are legitimately in fear for your life or serious bodily harm. If he pulls a knife of gun out on you while in the act. Etc. only you can make that judgement.
Also, in a beach...they're a lot of people for a long long stretch of open area. Any of those bullets that you miss, are going somewhere. And one better pray to God that they're going where you intended them to go and not anywhere else.
A tactic I was taught and learned (by the way is so much easier with soft sand under you) is either kneel/crouch and aim up at the assailant. Or drop down and spread your legs and open fire on the assailant. Thus changing your position as a target and aiming up in case any bullets are missed.
^ that takes a good amount of practice.
Anyways. Hope this helps.