Just want to drop a line and say that I've dove with Low Key for years now and they are a solid op. As far as "bigger sea life" goes, depending on the dive site you'll probably see tarpon, green and hawksnest turtles, stingrays and possibly an eagle ray or nurse shark.
Having spent plenty of time on both St. Thomas and St. John, I greatly prefer the latter - way less built up (3/4 national park), and you'll find more of that local/out of the way feel there than you will on St. Thomas.
Skinny Legs in Coral Bay is probably my favorite 'local' restaurant. Also a fan of Margarita Phil's (Mexican) in Cruz Bay. If you feel like going high-end, Asolare has amazing food and undoubtedly the best views of any restaurant on the island. There is also an awesome smoothie joint on the side of the road in the middle of the island - I am blanking on the name.
Honestly, the snorkeling on St. John is frequently just as good as the scuba diving. Waterlemon Cay (in Leinster Bay) is probably my favorite for snorkeling, though I love Salt Pond Bay as well - I always see multiple green sea turtles, stingrays and great barracuda there, as well as reef squid and the occasional spotted eagle ray. I enjoy Hawksnest Beach, Honeymoon Bay, and Jumbie Bay. Trunk Bay is lovely, but go after 3pm to miss the crowds.