First and foremost the dog has to want to swim. Like another poster said they have a golden who does not care for it.
My suggestion is get your dog REALLY eager swimming, chasing and retrieving from the waters edge. Dock jumping is ALL about confidence and drive. Once you have that working well, you can introduce a dock.
Now some dogs as noted just fly off, others well they get a bit suspicious. The way we start newbies at dock events off the big dock (competition docks are 2' off the water) you keep the dog close to the end of the dock, then toss the toy no more than 6' away "an achievable goal", and let the dog jump. Do that a few times, as the dogs confidence grows, you can move back on the dock and throw farther. However until the dog is really confident I don't recommend the full length dock sprint. You don't want them to get scared.
Now some dogs have a style I called "stop and drop" not much you can do if they get more confident they may learn to leap, but some dogs like that style. Basically they run down the dock as fast as their legs can carry them, the stop at the end and just short jump into the water. That was how my Maxine jumped her entire life. You would think by the speed on the dock she was going to blow it out of the park. Stop, small jump. LOL
Dock jumping is about having fun. Dogs do not know if they jump 6" or 30', so only people can get upset about that. Also it is a hard sport to train for improvement as the dog gets rewarded by the toy, whether they jump well or not. So keep it fun, and they will give you the best response!
Dock Dogs, Splash Dogs, and Ultimate Air Dogs are all venues around the country where you can play and compete. They all have websites and forum boards. You will meet a lot of fun people.