i've dived isla de la plata aug/septembers in 2010 and 2011 as part of a conservation project. at that time of the year its amazing, its where the largest aggregation of giant manta rays in the world takes place.
expect conditions similar to the galapagos, cold water, not very good viz (40ft ish) and crazy currents (very extreme at a couple of the sites), add in the illegal fishing that goes on everyday dragging lines right above you as you dive and you get the picture.
i dived with exploramar exclusively on both trips, they're very good. great dive masters and michel the owner is a great guy (he pioneered diving at la plata by bringing tanks down from quito for years until he opened exploramar a few years ago) and very knowledgeable (he's also a marine biologist). there are a couple of other dive shops in puerto lopez who visit isla de la plata sometimes but from my observations, safety is extremely lacking and its my understanding they don't even have permits to be diving there.
i highly recommend the diving at la plata and exploramar as your guide. i shot this video at isla de la plata last september 2011.
if you need more info just ask, i love talking about ecuador.
[video=youtube;H0j8LLyrJdo]http://www.youtube.com/watch?v=H0j8LLyrJdo[/video]