Is this really a dog park or just a public park? If there are leash laws in this particular area and he still lets his dog run off leash then you can tell him to get control of his dog before you call the proper authorities. Have a cell phone handy with animal control and your local PD's phone numbers on speed dial. Let him know you mean business. Just because people let their dogs run off leash doesn't mean they can. I like your super soaker idea, I would use that one if I were you. Load it with something stinky or sticky. If the owner has a problem with it, squirt him too. You've tried being nice, now it's time to get nasty before something bad happens and this dog has to lose his life because his owner is an idiot.

I used to take my dogs to a park that requires dogs to be on a leash (signs were posted) but people still let their dogs off leash anyway. This one Lab comes running up to my dogs, who were on leash, as his owner is walking several yards away getting even further from her dog. So she starts calling to the dog who is progressively harassing my dogs (they usually like other dogs). I yell at her to come get her dog. She continues to call to him as my dogs are getting further agitated. Again I yelled at her to come get her dog. She takes her sweet time walking over still calling to the dog who is not listening. Finally she gets to us and calls to the dog instead of grabbing him. I told her she needed to get her dog on a leash and that there is a leash law. She still tries to get her dog to come to her by calling it. She points out the fact that everyone elses dogs are off leash and I point out the fact that my dogs are Pit Bulls and if her dog pisses them off or starts a fight with them that her dog could lose and me and my dogs would get the blame. So she gets her dog and tells me I need to be on a leash. I had to hold my boyfriend back from kicking her in the a$$ for that remark.
Point of the story; it could have turned into a disaster that could have lead to some lost lives because of a careless owner. This guy's dog is going to run up to the wrong person and/or their dogs one day and some lives could be damaged if not lost.