Well I think Matt could have helped, cos he was there a few weeks ago and maybe others could too. But I'm afraid your question has come on a bad day. Were all getting banned from here tomorrow, so all a bit upset. But Kim is sure to know in our absence so ask him.
The usual moorings sunsail operate around there, bt not specifically St John's AFAIK. all on www. Or do a google search. Sorry if you knew that and were looking for very specifically St Johns or a one man/one boat or non-advertised source, in which case I can't help. I have used moorings out there for 44 foot Beneteau stuff, they're fine though imho best to pay the small premium and get a boat less than 1 year old so not too busted. I can sell you a C-map chip 1/2 price if you want, be quick before someone else snaps it up though..... ;-)
Antigua very wavy, better for sailboats, not power. Dunno if sail or power is what you're after. There are cheapy flights to the BVI, a better short-distace cruising area and more small islands, rather than one big one. The main sailbase is in the south, and sunsail with sharholder loot has bought some/most competitors in English harbour.
For powerboats, but again in virgin islands, mor protected, more to see, one hour flight by LIAT airline ...there is VirginTraders (again, get contact via Google or yahoo) with yank-style 48-foot flybridge boats .
Again, sorry, can't help direct with knowledge of St John's chartering. It's a bit of a dump, so if you did charter there you'd want to clear off