Now that he jumped and was able to get back aboard, I would think he may do it again. I don't think you can train him not to do that. However, you can train him not to get up on the sides which would probably keep him from jumping over.
My dog has not jumped. We did "push" her off a couple times so we could swim to the beach.
Always keep the life vest on her. It also makes it easier to pick her up.
rkc