We took a 6 hour tour of Isla de la Plata, organised through our hostel, Itapoa, but actually with Machalilla Tours company. We had an excellent guide who explained a great deal about the Blue Footed Boobies and the Frigate birds as we took one of the shorter walking routes (the Blue route). We then went snorkelling in one of the small bays on the eastern side of the island and were able to look at the coral reef, as well as seeing rays, sharks, eels, tortoises and many beautiful fish.
Our guide told us that there is more activity from June onwards as the birds will be mating and the whales come to breed near Isla de la Plata. He could speak good English and was happy to answer all sorts of questions about the birds, rainfall and guano!
We would highly recommend visiting Isla de la Plata, but it sounds like the experience is even better in June through to September.