Make sure the body of water you are attempting to hydroplane is smooth and still. To get your dirt bike to hydroplane, you need to get propulsion and forward movement as well as get started smoothly. Choppy water will end up in disaster.
Map out your runway. You will need to have long and level runways on both ends of the body of water you are trying to cross. It is better to err on the side of caution here since more space and longer entrance and exit runways will lessen the chance of an accident.
Ride your bike around and check out your surroundings. You need to be going at least 70 mph for this trick to work. Make sure you can get the dirt bike up to this speed by the time you reach the water's edge.
Kick it into at least fourth gear when you are ready and head straight for the water.
Keep your center of gravity low and just barely behind the center of the bike. Just as you get to the base of the water, push your heels into the pegs to get your feet secure so they don't blow off the pegs once you hit the water.
Slide back to the end of the seat when you hit the water. Wind out the rest of the throttle all the way. At this point, it's either skim or swim.