Before we got our trailer, we were boaters. We had to put two giant stickers, one in the cockpit, and another on the transom.
On ski/wakeboard boats, there is a swim deck on the transom, and there was a sad story of a kid who died because the father allowed him to "body" surf behind the boat, while hanging on the the swim deck.
So we had to be warned that exhaust has carbon-monoxide, and it kills. State of California mandates that ALL boats must have this warning now.
FYI: Transom also comes in handy for mixing drinks at the campground!