As noted, you arrive in the Galapagos via airplane. When I did my trip, I went via Guayquil and left via Quito. As far as I know, there isn't any camping allowed in the Galapagos, so you'll have to do lodgings. I only spent a couple of hours wandering through Puerto Ayora, but it seems like an o.k. town, and there were plenty of operators offering scuba diving and snorkeling. I went in November, and most of us wore wetsuits against the cold water. It gets hotter during the summer, so things are a bit more comfortable for swimming. Otherwise, since Ecuador is on the equator, there isn't really a bad time to visit the Galapagos.
Most visitors arrange for a live-aboard boat tour of the islands. I did an 8-day cruise with Lindblad. There are many boats offering cruises of various lengths and for various budgets. Some people get lodgings in Puerto Ayora and then do day tours to visit various islands, but you tend to use up a lot of time traveling to and from islands versus living on a boat and traveling overnight.