They often are. Still, the question is always how bad they are per tourist, and I suspect that the solo sailing folks aren't much better in that regard.
Crew/passengers on a small sailboat will use less fresh water in a day than a cruise ship passenger uses to flush the toilet once.
They will know exactly how much water they used down to the quart, same with diesel. They will have very tight energy budgets as well and track it by the watt hour. Their energy will likely come from renewable sources.
Instead of daily hot showers, on a small boat you get a cold salt water shower every few days with a pint of fresh water at the end to rinse.
