This is one of the more interesting topics for me. It is funny how I once had a steadfast rule on human aggression and as I got older it lightened some, maybe even lightened quite a bit.
A number of years ago it was anything that I thought may evolve/grow into a man-bites/human aggressiveness was given a vitamin that prevented that behavior right there on the spot. I mean 'zero-tolerance' in every sense of the word.
As I got older and more experienced I began to have more latitude with the very few dogs that show some signs. I started to try to figure out the "why's. when's and what for's" and if those were not going to end up with me being bit then it became a 'no harm-no foul' mind set.
I have only put down a couple-three since the early 90's. If those same scenarios popped up today it would only be one. As the others would have been 'avoidable' now but at the time it was that 'zero tolerance'. The one was a Jethro/Mayday bred male. Lots of times I feed in the dark due to work schedule. He was fine, out going, super puppy til he was about 16-17 months. As I came thru his spot he grabbed by pants leg/top of my boot and I took it as playing. I brushed him off, fed him and all was well. The next night he put his mouth on my calf as I cleaned and filled his water bowl, slight pressure just enough to let me know he had an issue with me. Again, I took it as playing as he was a springpole/tug maniac. The third night he met me at the end of the chain for 'no apparent' reason and would not let me in. I mean teeth showing, snapping, etc. etc. I then fed him the vitamin that prevents all human aggressiveness.
To the original poster, yours is a simple fix. If he is worth keeping, keep him on a lead so you are in control. Period. Private beach or no private beach (and I have never seen a private beach that had fences to the water) pit bulls must be restrained 24 hours a day. That may be a chain, a kennel, a fence, a crate, your front door, but there has to always be a barrier between the dog and things he can hurt. Every bad story starts with, "He never did anything like that before", or, I let him run free for a few minutes everyday". People quickly forget these dogs were bred to basically kill shit, other dogs mostly, and there is no magical switch that turns this off and on at the owner's whim. Sometimes these dogs will re-direct their 'skill set' without warning and with the ever most subtle of signs. Barriers prevent the re-direction from being on the evening news.
I would simply keep him away from the things he has possessive issues with, thus a six-foot lead is the cure for that ailment not the vitamin I spoke of. EWO