A few that have worked for me...
Unless you want to try to explain what can and cannot go down a marine toilet--and why--to landlubbers...or worse yet, have to unclog because you didn't explain it, it's best to tell them that NOTHING goes into it they haven't eaten first. I always kept a "diaper pail" with lid and trap door, or a step-on-the-pedal waste can with lid and liners in my dock locker...when landlubber guests were expected it went into the head beside the toilet and instructions to guests then become simple: unless you've eaten it first, it goes in the pail. NO exceptions, not even a toothpick. (You can make a sign to hang in the head if you're really uncomfortable telling them). When the guests go home, the liner goes to the dumpster with the rest of the trash and the pail and sign go back into the dock locker.You really DO need to explain to guests how to flush the toilet, but a lot of people seem to be uncomfortable about how to approach that. The easiest way is to make it just another part of your "this is where everything is and how it works" tour of the boat--something you should do for ALL first-time guests, landlubber or not (how many times have we all had to hunt for 10 minutes for the light switch in the head on a strange boat...and finally had to ask?)--when they come aboard. The toilet just becomes part of the "tour," and a LOT less embarrassing for you OR your guests than having to show someone how to flush a full bowl.And by the way...I've always made safety issues a part of that tour.. "Here are the lifejackets. God willing we won't need them, but just in case, here’s how to put one on. The light switches are located...and they work by…. The water faucets work just like the ones at home, but please don't let the water just run, because what's in the tank is all there is. Please don't try to help unless specifically asked to do something, you could get hurt if I zig when you expected me to zag…" etc. It's been my experience that landlubbers are a lot more comfortable aboard immediately when they know a little bit about the boat, how things work, and what's expected--or not--of them.