Living in Bermuda, I've used most the operators and they're all quite good. You'll probably save money contracting directly. Dive Bermuda has two operations: one out of the Hamiltion Princess (1/4 mile from where the Zenith will be docked in Hamiltion) and one out of the Southamption Princess (30 min taxi ride from Hamilton). The Hamilton operation usually goes out around 12PM, the Southamption operation around 8:30 AM. You should call Dive Bermuda and confirm which operation is going out when. As for St. Georges, the only operation out there is Triangle Diving, operating out of Grotto Bay Resort (5 minute taxi ride from St Georges).
As for teaching your wife to snorkel, one of the best places is Tobacco Bay, in St. Georges. It's a 2 min. taxi ride from the dock, or a 20 min walk. Walking to it, you can stop at the "Unfinished Church", which is nice. The beach is kind of small and get's really crowded when the cruise ships are in, but for snorkeling it's great. No coral but usually some very large parrotfish and interesting rock formations.
Another (better) snorkeling suggestion is to take one of the snorkeling charters, out of Hamilton. All the good snorkeling in Bermuda is really only accessible by boat. Jessie James Cruises is really good: they start you off at a nice shallow calm area for snorkel #1, then a reef for #2, then a ship wreck for #3. They have a good website to give you more info.
