We will be going to GB for the 4th to the 8th of July. On our previous trips, we have stayed at Xanadu and dove with Xanadu dive. It is close to the Freeport/Bazaar/Casino area. This time we are planning to stay at Running Mon (near Xanadu) and dive with Xanadu. They offer several trips daily, have nitrox and do multi-dive discounts. They will also let you store your gear in the dive shop and give you a 10% discount if you book online. They also do the best shark dives.
We checked out staying in Port Lucaya and diving with Unexso. The Port Lucaya area is generally newer and the market place is fun, but I don't think the Casino is open yet. The Our Lucaya complex is impressive, but quite pricey. The cab rides between Freeport and Port Lucaya aren't too bad (about $10 each way) Unexso is a well established operator, however, they only run a very limited dive schedule (I was shocked when I talked to them as they only have one boat a day on many days) They don't offer nitrox and are a bit more expensive than Xanadu, but they do offer the only dolphin dive around, however, they are proud of it as it is about $190.
As for flights, your best bet is probably Continental Connection. If they don't have a direct out of Tampa, I believe they do a direct from Orlando. The most you would have would be a connection in Ft Lauderdale.