My GSD and Border Collie both compete... we are usually in the water when it's warm!
I would suggest building a lot of drive in a floating toy, and starting from a small ledge/dock into the water. Get him very amped up, and throw the toy very close into the water, just out of reach for him bending off the dock. Encourage him yelling things like "get it get it get it" and such. Keep up the excited talking. Hope he jumps/falls in.
If he's a very confident dog that doesn't startle and recovers quickly, maybe give him a small push if he's right there anxious for the toy and just can't quite make the leap on his own.
If you can find another dog that jumps to "show" him how to jump, that would be ideal, then use my prior advice as a back up plan.