hitchhiking on yachts
No and No, to your first two questions. As to the last, it could be just as dangerous or as wonderful as any other method of meeting someone new. I don''t think the Internet plays any role in how that would turn out.
You need to use the same evaluation skills you would use when deciding to do anything with a stranger. Depending upon where you sail, and how long between ports, you may have far fewer options if things go bad than a land-based relationship close to friends and family.
Good luck with your decisions and possible adventure.