Sailing boats are not always very glamorous.

The non return valve on the toilet wasn't working, so water in the outlet pipe going up to the top of the holding tank would flow back into the bowl. As long as you'd flushed thoroughly, just meant water in the bowl could slosh out under sail, but if someone didn't flush thoroughly enough... Suffice to say it needed sorting out!

The valve is under the bowl and can only be removed when the whole toilet is removed. Grrrr. Fortunately, as I'd expected, the problem was scale. So remove the valve, put it in a container of kettle descaler for an hour, rinse and refit. What a lovely way to spend a sunny, windless, Sunday.