We did the Peter Hughes liveaboard in June '02. Its advantage is that you get a 10 day, rather than 7 day, trip and you get a fair number of land excursions (marine iguanas, giant tortoises) as well as snorkling with penguins.
It is an incredible experience. When it was over, I was exhausted but exhilirated. We went with two other couples with whom we regularly dive. We all said it was a once in a lifetime experience -- and a week later we all were talking about going back!!
As far as water temp, conditions etc. It can vary, both by time of year and where in the Galapagos you are diving. When we were there (June), the early part of the trip was pretty chilly -- as little as 60 degrees at the surface. By the time we got to Wolf and Darwin, the water temp was rising into the lower seventies.
I was chilly at times, but managed to get by with a seven mil suit, 5 mil gloves and a 3 (I think) mil hood. And as has been mentioned, it can be rigorous at times, particularly when you're going against the current.
But you gotta do it....its unlike anything else I've ever seen.
Seth