Difficulty: Moderately Easy
Step1
Don't panic, and don't yell at your first mate--these things happen.
Step2
Turn the motor off and pull up the motor and propeller or outdrives.
Step3
Attempt to determine if you have damaged the hull. Assuming the answer is "no," proceed. (If you put a hole in the hull, leave the boat where it is and use the emergency channel (16) on your marine radio to call for help.)
Step4
Ask the strongest and heaviest person (or persons) to hop off the boat. The water is probably pretty shallow, so they should be just fine. Someone should hang on to an attached line so the boat doesn't get away from you.
Step5
Send 1 or 2 small people to the bow to put weight on the front of the boat and lift the stern a bit.
Step6
Gently rock the boat until it is floating freely. Once the boat is back in deeper water, invite everyone to get on board again.