As for the fish/snorkel/shore lunch, contact Hustler's and tell them you want to rent their Catamaran for the day. Our guides cooked us an amazing lunch on the beach, and we had a fantastic time. They'll take you wherever you want to go.
It's low season right now, so I'd just wait to book trips when you get there. I don't know who Banyan Bay uses for their tours, but I'm sure whoever it is, they're good.
Bug repellant -- anything with Deet.