I know this thread is a little old, but I figure someone might read it in the future trying to find info about Horizon.
I've never chartered with them, but I spent a few days in Nanny Cay Marina where they are baised a couple years ago on a trip through the BVI while I did some repairs on our boat. I had a few dealings with them while trying locate places to get parts and tools over the course of the weekend I was there.
They were very freindly and extremly helpfull in getting good advice on everything from the local taxi's to calling west marine and asking a favor from the girl working to stay open a few minutes extra so we could get our badly need parts that night (it was a sunday).
If they take care of their boats and customers as well as they do other sailors in the marina, I would definitly reccomend them.