Having done 10 or 11 trips through the Grenadines, leaving out of St. Lucia in the last 10 years, we've never had an issue except for a crazy banana man in Marigot Bay (my fault, I threw him a dockline as we were preparing to dock at Chateau My go and then he wanted a obscene payment for his service, ended up just giving him 10 EC). You do have to lock your dingy, motor and gas tank at night and whenever ashore.
We usually make Cumberland Bay our first stop in SVG, it's much nicer than Walilabou (sp), more restaurant choices, nicer boatboys, and more beautiful bay.
One mistake people sometimes make is when the boat boys are showing their wares, instead of just telling them they are not interested they say "maybe later/tomorrow/ ect" and then when the guy returns they feel they are being pestered.
We always have a guy tie our stern line ashore for us and usually pay 10 EC, that buys a lot of goodwill.
I've got several trip reports here on our Grenadine charters

Drink all day at home, your friends worry about you; do it on vacation and they say "what a good time you're having". Save your friends needless worry, travel more!