When I am looking at boats in yards I don't think it's a problem to be a approached and asked my intentions, in fact I see it as a good thing. In the yard I worked at I would always keep an eye out for unfamiliar vehicles and note the plates down. I would have no problem approaching strangers either.
We had stuff stolen from the workshop and the yard manager saw the van during the theft but thought it was me (common transit van) so didn't investigate further. He didn't make the same mistake twice. They had the cheek to return a few days later and this time he locked the gates and trapped them inside.