dog behavior in water

Question
Hi
We have a 21 month old purebred black lab. We are his third home. Great big lovable fellow, high energy. He isn't aggresive, but he isn't submissive either. He humps about 1/4 of the dogs he meets. He is neutered. My problem is that he sometimes will chase after a stick or toy in the water that another dog is swimming for. When he gets close, instead of getting the toy, he tries to mount or otherwise climb on the dog! This usually causes the other dog to go under a little and loose intrest in the toy, scaring the crap out of their owner! He doesn't really respond to my yells to "OFF" and he seems to only do it with some dogs.
How can I correct this behavior? We live on an off leash lake park and otherwise he plays well, if not a little too exhuberant at times.
Thanks

Answer
You may be seeing one of the behaviors for which this poor dog has been passed around.  Labs are extremely high energy, large dogs and that combination is difficult in the first 2 to 3 years of life in some of them.  These dogs need a job.  I suggest you use positive reinforcement training (away from distractions and then brought into more and more populated areas until it works in the park) and train this dog solidly for what he is bred to do:  retrieve.  Capturing this behavior should be quite easy; bringing it along to a solid, 100% working behavior should not be difficult if you do it right.  Go to karen pryor's web site and learn about using the clicker in training.  That is the "cue" that reward is coming and can be given (once it's used to train one solid behavior) as an instantaneous communication to the dog that its immediate behavior IS RIGHT and will be rewarded.  Most dogs look for the reward immediately after hearing the click.  If your dog is trained to retrieve only on command (for click and large special food treat), his voluntary behavior as you described might begin to self extinguish IF you give him the signal (click) as he approaches another dog's stick but TURNS ASIDE at your voice prompt (which could be anything except whatever you've used so far.)  That voice prompt should be used randomly throughout the day at home.  The dog needs to associate his LOOKING AT YOU with that voice prompt and then a click/treat: in other words, dog is lying near sofa, you use prompt (let's say the word "crisco"), dog looks AT YOU, you click, treat.  Repeat randomly.  Conditioned response to "crisco" is acquired, must be absolutely 100% reliable.  Now yell "crisco" if dog is voluntarily pursuing another dog's stick (and he is 100% reliable in fetching for you on command).  Dog WILL STOP and look at you: click, dog should swim back to you for VERY LARGE treat.

This all takes some time; humping is a self rewarding non-sexual dominance behavior.  But this dog clearly doesn't have signal from you that you are higher ranking and training will do that.  After you have the retrieval behavior and the "crisco" response, your dog will be ready to learn more.  There's no end to the things you can teach a dog (providing its natural behavior) and the more the dog learns, the more able he will be to make appropriate choices.