I've done a fair amount of volunteer work on board the Hudson River Sloop Clearwater, where before we leave the dock the Captain gives a safety talk. Don't lean on lifelines. Face the ladder going below. Here's where to find a life jacket and here's how to put it on. Here's where it's safe to sit. Here's where it's not safe to stand.
We don't sail with guests very often, but I try to remember at least the basics above. I usually add two more: Don't pull on a line if you don't know what the other end is attached to, and please don't untie a line if you don't know what the other end is attached to.