Certainly true about head procedure. So I've posted written instructions next to the toilet. That helps but is not a perfect solution.

I once had an inexperienced guest accompany me on a 45 mile trip. Did the usual briefing, including showing him how to use the toilet. It was cold (in the 40s F) and we were pounding into head seas (3'-5' chop) all day. He lasted an hour before he got seasick. The trip took almost 12 hours and he ended up spending most of the trip below wrapped in a sleeping bag on a settee when he wasn't visiting the head. I was pretty much stuck in the cockpit managing the boat. When we reached our destination, my 17 gallon holding tank, which was empty when we started, was full. I have no idea what he did.

Also, in my experience, few guests remember about the boom swinging across the cockpit either. Or radio procedure. Or much else of what I brief them about.

So I have also posted written instructions about how to make an emergency call on the radio and how to start the engine. And I keep an eye on their heads, fingers, and other body parts and forewarn them when necessary.

I also keep extra eyeglass retainers on board and offer them to guests. And caution them about securing their hats. (But I've still had to do some MOB drills which, alas, have not always been successful.)