Some of my briefing includes:

1) Wear PFD if you leave the cockpit while underway (I wear an inflatable belt all the time underway, so I'm ready to move quickly if needed).
2) Explanation and warnings about the traveler and boom, and fingers in winches
3) Rules for using the head, and explaining need to use water conservation techniques when washing.
4) MOB procedure (who does what; mostly I want guests to throw a float and keep an eye on the MOB)
5) I ask guests whether they want to be treated as passengers or crew. Crew get introduced to the control line placement and proper use of winches and clutches. I may also explain the engine and AP controls if it's going to be a longer cruise.