Don't underestimate how many dives you will want to do on the Coolidge, the thing is massive, you could easily do 7 days straight and not see it all. The dives are deep but short, so you won't be doing more than 2 dives a day.
I went with Aquamarine late last year, and lets just say that I rapidly bailedand went to Allan Power - as far as I know he only does the Coolidge these days, but it would be easy to ask. Be warned that Aquamarine may not have what they advertise on their website - when I was there they had lost all of their dive staff apart from the owner and the locals. All of their Tec and PADI qualification that they claim on their website simply didn't exist. I'm told their main instructor had left to set up his own shop, but I can't confirm that. What I do know is that they were extremely disorganised, overbooked, and had death-trap equipment for hire.
Million dollar point is nice, but i'd suggest doing it before the Coolidge, otherwise it might be a dissapointment. There's also some nice reef diving i'm told, I did one quick dive but that was it due to being out of time.
Where are you planning on staying? I cant rave enough about Deco Stop - cheap as, great hosts, beautifull, and some of the best food in Santo.
Bokissa Island is a 5-star alternative, although I got very bored with the same buffet food everyday. They also have their own dive shop, but I didn't end up diving with them. They seemed fairly organised though.
