If you or your vessel are in immediate danger, hail the Coast Guard over VHF Channel 16 using the standard Pan-Pan or Mayday call depending on the urgency of your situation. This may also bring nearby boaters to your aid.
As you might imagine, PWC operators face different problems when they run aground. In many cases, an operator can generally get off the PWC, and push/pull the boat off of "the hard".
PWCs are easier to examine for damage--make this a priority when you ground. Check the hull both inside and out for cracks or leaks. Also check that nothing has come loose, and do the "sniff" test for gasoline. finally, check the impeller intake to make sure no debris has entered the jet drive before heading out.