So there's a couple of schools of thought here...
1) the person that wants to dive with people they know
Some people like to book with their LDS, and dive with others that they know, are familiar and comfortable with. A good LDS often puts together a couple of trips a year, and it is a great opportunity to travel, dive, and socialize with folks who live in your area. I've personally never found the deals to be THAT good, but there's a trade off if you value the social aspect of it.
2) the person that wants to book a trip, and have their hotel/air/diving all included in one price, but maybe is looking for a better deal than their LDS can offer
Check out
Islandreams.com
Caradonna Dive Adventures - Dive Travel and Vacations
those 2 aren't bad.
3) the person who wants to get there and stay there and dive there for the least amount of $$ possible (ME!)
so - this is how it works for me...I scour multiple sites for the best deals on travel (air and hotel)
Continental Airlines Vacations Home
Travel: Airfares, hotels, vacations, cruises, car rentals and more at Travelocity.com
American Airlines Vacations - All Inclusive Vacation Packages, Beach Vacation Packages, Family Vacation Packages
Best Fares.com :. Cheap discount airfares, Cheap discount flights, Cheap discount tickets, Cheap discount cruises
Alot of time the biggest expense is airfare...and the airfare is USUALLY (not always) cheaper if you live in a hub city like Denver, DFW, or Houston, if you travel on the carrier that travels out of that hub, additionally you usually can fly direct. I live in OKC, OK - not a hub city, I have to stop in dfw or houston for everything. At any rate - figure out if you are a hub city, and which airline you are a hub for - check that site for airfare alot. Usually I find it cheaper to book airfare and hotel together on one of these sites.
Diving I pay for seperately...I pay for my diving (primarily Mexico) in cash when I get there. I find that if you can buy 2 tanks for 70 bucks for 2 tanks online, you can pay closer to 60 in US dollars (CASH, not credit card) when you get there. If you pay in credit card - it always costs a little more. The smaller the shop, the more this tends to be true. I like the smaller shops anyway. For instance PapaHogs in cozumel is about 60 dollars for 2 tanks if you pay cash, in person - if you bring a credit card, they'll charge you 15% for "fees and taxes".
There's lots of deals out there for cozumel right now...I can go all inclusive (meals drinks flight hotel, everything but diving) for the week to a smaller hotel in cozumel from OKC for a week in July for about 700.00 per person. I was looking at Cancun for July a month ago - closter to 600.00 (but you'll like diving cozumel more). I like the all-inclusives, because if I go out to eat every night downtown, I'll spend more...and my wife can have as many cerveza's as she pleases...but if your pinching pennies...don't go all-inclusive, and eat at simpler, less expensive resteraunts while your there.
Depending on who you are and what you want - it may just be easier to call up a dive travel agency like island dreams or caradonna and get a quote and pay what they offer...but I'm a bargain hunter - so I scour the internet and look under every rock I can find...sometimes I score big, sometimes I don't.