Unfortunately, this is not something you can prevent unless you build a fence around the pond. Your dog evidently has strong instinct to be in the water and he enjoys it. You can't fight instinct. However, attitudes and training can help. If you look at it from your dog's point of view, he's confined to the yard without much to do.
Unfortunately, punishing him each time he goes in the water doesn't work. He is not human, so he can't understand the reasons why you don't want him there. And if you get all excited each time, that may make him want to do it more. Dogs like emotion, even if it is negative.
Here's something you might try: Get a kid's swimming pool (Kmart, Target or Wallmart) and show him that this is the place he can play in the water. Fill it up and put it in the yard where it won't bother anybody. If you catch him near the pond, don't say anything, take him by the collar gently, direct him to the pool and praise him wildly and give him treats. Then play with him there. Make the pool the cool place to be. You could even put some rocks in there for him to play with. This is the same thing we do with diggers. We build a sandbox and tell them they can dig there, but nowhere else in the yard. Their instinct tells them they need to dig... or play in water; then we just find a place that it's ok to do that.
My suggestion if you don't get a pool, is to keep your dog with you as much as possible. Take him in the house whenever you are there, but keep him on leash. Tying the leash around your waist will keep him close and well-behaved. Talk to him and put words to everything you want him to learn. Being together in the house all the time, will cause a strong bond to develop, and he will perhaps listen to you better. Try not to leave him alone any more than you have to.
It's time to sign up for training classes. If you can't find a class, try a Petsmart or Petco. If you aren't near any of those, go through How to Love Your Dog, follow training instructions, and do the Lesson of the Day (Lesson of the Day) several times a day.