Just tell them, "I just want to be friends", before they get onboard. Although the sign, "No, I DON'T want to sleep with you!", Inland girl", wouldn't hurt either.
Just think about the situation. Here is a married man doing the sport he loves the most with another woman, ....where's the wife??? The fact that your sitting there in her place on a boat alone with a man, (her man), and she isn't! Pretty much says it all right there. If advances are unwanted, (the man is expected to move first and take his chances), You'll just have to make it clear upfront. Don't worry we're used to disapointment.
The Sun has Risen on a New Day filled with the Promise of Adventure.