I guess communication is the key here. And do some vetting before getting on a boat with a stranger. Dating on a boat is going to be a little awkward no matter what, because there is no escape, (and long hours at sea can make anyones social skills rusty). There are plenty of opportunities for miscommunication, 1. She thinks I provide decoration/companionship he can do the work. 2. He thinks here a a female that is here for a "ride". 3. I may be looking but your not "it".
Unfortionately social rules make this topic a cat and mouse game from the beginning, thats half the fun. Being up front and firm if no relationship is desired is the best policy, (these days either sex), ... if you are interested continue being coy. But on the other hand why would any self respecting adult want to spend months alone in a small floating box with someone you didn't like???
The Sun has Risen on a New Day filled with the Promise of Adventure.