As always, try to remain calm and think things through in an emergency.
1 - Never leave your boat in gear and step off, even for a second, she's faster than you think and will run away.
2 - Never voluntarily go in the water fully clothed, chances are, no one is THAT good of a swimmer.
3 - Keep your ropes in good order, don't let them trail in the water
4 - Never ever swim up to the business end of any boat that is in gear. The prop will pull you, catch you, mangle you.(see point 2)