I am going to reply more fully re Galapagos diving in the Cocos vs Galapagos thread, but I will say that I have been to Galapagos three times, all via live aboard (Aggressor). Assuming you are hoping to see whale sharks, and schooling hammerhead and Galapagos sharks, which for most are the siren call of the Galapagos dive experience, you are likely only going to see these populations up at Wolf and Darwin Islands, and the diving at those islands is only accessible via live aboard vessels. If you are going to go live aboard, you should get confirmation from the op of the planned itinerary. There are live aboards that operate in the Galapagos that do not go up to Wolf & Darwin. That would be a mistake, in my opinion.
In answer to your question, there is a big difference in sea life between the north and the south. To see it all, you should dive it all.
You do not need to do four dives a day to capture the experience, but having travelled all that way (and spent the dough to be there), if you were doing a live aboard, you would likely take advantage of most of the opportunities to dive. Diving from a live aboard is so much less strenuous than land-based diving. Suit up, climb into the panga, a short ride to the dive site, back roll in, enjoy your dive, surface and get picked up by the panga, short ride back to the mother boat, peel wetsuit, hot shower, towel off, have a hot drink, a meal, a nap and then do it all again. No gear schlepping required. Set up your gear once at the beginning of the trip, break it down at the end, and you are good to go.
Land-based diving is done out of the more southern islands (Porto Ayora has more ops than you can shake a stick at). You will likely see sea lions (definitely a highlight of the Galapagos experience), and if you are lucky, at Gordon Rocks, you might see a squadron of rays or even an odd hammerhead or manta blowing through, but I think it is highly unlikely that you will see whale sharks nor schooling sharks in that part of the Galapagos. Cousin's Rock is also a world class dive (doable with land-based ops), renowned for its tiered walls covered with black coral bushes (which are actually yellow), schooling barrracuda, sea lions and wonderful macro life (sea horses, frogfish, nudibranchs etc).
Also, you should be aware that the water temps in the southern islands are significantly chillier than the temps up at Wolf & Darwin, at least in my experience. I have seen temps as low as 58F at one of the southern sites, with the norm somewhere around 65-74F. In the northern islands, temps have run between 74F and 80F, sometimes on the same dive ;^). All of my trips have been in late August/early September, which is prime whale shark season (which, if memory serves, generally runs late June through September/early October). These prime whale shark months are in the Galapagos winter, so water temps are likely quite a bit warmer during Galapagos summer. Winter is also typically quite overcast, which, when combined with murky water, can make for challenging wide angle photography.
Also, whether you are diving via live aboard or land-based, you should be aware that the diving conditions in the Galapagos are generally quite challenging - oftentimes significant current, surface waves, big animals, murky water and cold water all will add to the complexity of the experience. It is not to say that newer divers can't handle it, but it is definitely more difficult than benign Carribbean or South Pacific diving.
I hope this helps. I understand that we are not supposed to post trips reports in this forum, but I do have a lot of stuff on the Galapagos on my personal website - http://www.awoosh.com
- have a look in the Galapagos Directory linked in the main site directory. Awoosh.com is a totally non-commercial website, the purpose of which is to share information and experience with other divers, so I hope the moderators do not mind if I direct you there from here.