Amanda,
Western islands are newer geologically talking, the landscapes are more overwhelming there than in the Easter islands, the water is very cold up in Isabela northern boundaries, so snorkeling there won't be a very good idea probably specially in October. The advantage of the north side of Isabela is that you can admire there the marvelous flightless cormorant at Fernandina and eventually penguins there and at Punta Vicente Roca.
In the south we have Gardner Bay snorkeling site at Española island, this is a beautiful place where you can see white tip reef sharks, mobulas, tropical fish amongst other great marine fauna, the terra firme at Española is fascinating, this island has the greatest diversity of bird fauna amongst the whole archipelago, there you can see Galapagos hawk, mocking birds, ground finches, frigate birds, blue footed boobies, Nasca boobies amongst others. Floreana has also a nice snorkeling site known as Devil's Crown, there you can see a bunch of interesting marine organisms and at San Cristobal you can snorkel at Kicker Rock where in a good day you can see three species of sharks in one session: Galapagos shark, white tip reef shark, black tip shark and hammerheads.
Make your best decision and enjoy the best marine fauna place on Earth!
Saludos cordiales,
Dennis Yépez
Guanguiltagua Expeditions