Tomek Wrzesinski has spent more than 20 years selling supersized boats to the uberwealthy, and he’s found one nearly foolproof way to get a client to bite.

It starts with sending a cryptic weekend text message: “ Maybe I have something for you. I just need to check if it’s still available. It’s an off-market offer .” The last three words are usually enough to cinch the deal. “After 20 minutes, you have an answer: Yes, yes, yes,” says the Monaco-based yacht broker.

Wrzesinski’s method takes advantage of his place in a discreet network of off-market brokers who help the rich purchase boats without them being officially listed for sale. Thanks to…