You haven't said what area you want to visit, and that makes a big difference. The main diving areas in Belize are the northern cayes, the Southwater Caye marine reserve, and the three atolls. Most people who dive the atolls travel there each day from one of the other areas, but there are well-regarded resorts and dive operations actually on the cayes.
Splitting your time as you say would be difficult if you were to stay on the atolls though, as often transport arrangements allow only for complete weeks. In any case, it makes more sense to do your inland exploring after you've done all your diving, as you should leave 24 hours after your last dive before you fly. You might want to reconsider that aspect.
The northern cayes comprise Ambergris Caye (San Pedro), the principal tourist destination in the country. Offers good quality diving on the barrier reef just 1/2 mile offshore, and has several very cheap options as well as lots that aren't so cheap! The other caye is Caye Caulker, 15 miles south but another world. Very quiet, generally cheap and popular with backpackers. Lots of dive centers on Ambergris Caye, just three on Caye Caulker.
One of the main centers for diving the Southwater marine reserve is Placencia. A long way from the reef so diving days tend to be long and quite expensive, but well placed for inland jungle tours. Probably people more familiar with that area will post here. You can also dive those waters from a number of small cayes inside the barrier reef; the best known budget one is Tobacco Caye, accessed by a private ferry from Dangriga.
From the northern cayes it is common to take a day trip to the outer atolls, either Lighthouse (which includes the famous Blue Hole) or Turneffe, well known for the Elbow dive site at its southern tip. From these cayes it is too far to visit Glover's Atoll, the third of the Belizean atolls (there is only one more in the entire northern hemisphere, and that is nearby in Mexican waters). That can be visited from the southern places I've mentioned.
Travelling within Belize takes surprisingly long and is surprisingly costly. You should think carefully about what you want to do and plan accordingly. My suggestion is that for a week you may already be planning too much. Remember that from the northern cayes it is possible to take day trips to the mainland, and that at Placencia you are already pretty close to some very wild jungle and notable Mayan sites.
It sounds as if you might benefit from looking at some maps and doing general reading. You have ample time for both, because although February is in our "high season" I do not expect any many places to be sold out next February. Have a look at
Belize Maps, Ambergris Caye, San Pedro, Caribbean and Central American Maps and associated pages, but don't hesitate to ask more questions here.
