As a starter to explore ferry services, go to the Ferryhopper.com and punch in your route searches. Note: air travel between islands is not generally good. Air travel is normally only feasible between Athens (or other mainland destinations) and the island in question. Some exceptions do exist – Rhodes Kastellorizo, Kos-Leros-Astypalaia, Thessaloniki-Limnos-Ikaria etc. but again you will need bookings weeks in advance.
Since St. John is part of the U.S. Virgin Islands, U.S. citizens can visit without a passport. There are quite a few bargains on both St. Thomas and St. John right now, so it’s a good choice for travelers on a budget. Snorkelers should not miss visiting Trunk Bay, one of the best snorkeling spots in all of the Caribbean for seeing amazing marine life.