check the tent
sorry to be obscure with the title, but my first job was as a counselor at a Boy Scout Ranch, where we received extensive training to do the obvious. For example, we were told that when a scout was lost, the first thing to do was check the kid's tent. And half the time, sure enough, the "lost" scout would be found fast asleep inside his tent.
So, the obvious thing you should do first is to google the boat's name. Half the cruisers out there these days are keeping blogs so that friends and family can monitor the vessel's progress.
Almost every cruiser has someone back home who collects mail and pays bills on a monthly basis. This is usually a close friend or family member. So, the second obvious thing would be to check with the cruiser's parents or children, as the case may be.
If the need is not urgent, and the cruiser left no apparent way--such as a blog--to be contacted, I'd suggest you NOT post the vessel name here. I suspect that most cruisers would resent this invasion of their privacy.