This is a very quick reply - i will answer more fully if needed, it is very late here and I am V. tired!
Also i have not read anyone's replies so if I repeat advice or go against anyone, please understand that it is unintentional. (My shift button doesn't seem to be working 100% either - please forgive!)

I am poo with names, so "the dog" will have to get us through here!

As I understand the situation you have a very large dog that is displaying territorial/protective aggression at 9 months.

Number one thing to do is to call your vet and get this dog neutered ASAP. There tends to be a worry in owners of larger dogs that castration will affect their growth - with decent nutrition and symathy for longer bones (don't let the dog jump in and out of cars/trucks, allow free running up to the point where the dog begins to show signs of fatigue and put in a lot of time in building the muscles needed to support the movement and stamina needed for the dog to grow into a healthy adult - swimming is ideal as a muscle toner and respiratory enhancer) there will not be a problem.

Dog parks, by their nature, are not the ideal place to take an adolescent entire male.

Very quick answer - get the dog neutered and concentrate on getting verbal control - the moment you have to step in physically is a crucial one....it is a reprimand for bad behaviour and should be seen as such by the dog. This is seen as very "iffy" in the current trends of dog training. There are, however, times when your dog needs to face the consequences of it's choices. Being a large, entire male displaying aggression of any kind your dog is expecting a consequence. If you drag him away telling him loudly, "No!" then he hears the leader getting upset, assumes it is for the same reason he is upset (another dog daring to think it has the right to a water source when he has clearly made it his own!) and is encouraged by the leader to become more vociferous over the water source.
That is, obviously, not your intention.
You need to do 4 things - get the dog neutered, avoid dog parks for a while, go to a training class (not because you need help to train your dog - but as an alternative socialising environment for the dog) and learn how to correct the dog when he goes against something you are SURE he has been trained to do.
Proper correction is a fundamental tool, is an inhertited expectation and a crucial experience for all mammals if they are to survive.

So - there is the short answer!!!!!!!!!!!!!!!!!!