Bwana, I didn't do as much fishing on St. John as I normally do in the islands. That said, St. John has the same fish as any other Caribbean island. You can DIY, but always better to get some local knowledge first.
As you spend more time in the islands, you will soon find as a whole that inshore fishing isn't really their normal gig. They're more into snapper and grouper and other deep water fish. Tarpon, snook, and other inshore species used to not really interest them... until they figured out there are a bunch of deep pockets gringos willing to pay good money to chase those types of fish.
Puerto Rico, for example, now has a thriving sport fishing community targeting tarpon. It's pretty funny really, because one of the hotter spots is in and around the bay waters where the bridge goes over to Old San Juan. Nobody ever bothered with it. Now guides are making a lot of money off that fishery.
Hit the Florida Sportsman BBS (like this one) and pose your question there, too. You should get some hits. Try a little google-fu, too. Has worked for me DIY'ing in MX and other islands.