A few thoughts:
I've done several of the shark feeding dives, and have seen relatively inexperienced divers on them. Some of the newbies seem to handle it very well, some were a bit rattled (above and beyond the rush most of the divers felt), and one saw the sharks in the water, from the boat, and refused to enter the water.
I'd suggest scheduling the shark dive for after she has done a few dives, to see how she does on a "regular" dive. Depending on how long you are staying, maybe postpone the shark dive until a later trip, or for a later day during your stay, not on the first day of diving.
While the shark feeding dives do have a high concentration of sharks, and they are very "up close and personal" it is not uncommon to see sharks on the other dives in the area. While there is no way to guarantee that happening, if it does, ask your wife how did she feel about it. Obviously, if she is at all hesitant, don't force her, as it could turn her off to diving in general.
If she enjoys diving, but is hesitant about the sharks, then switch that dive for a "regular dive".
I'd also suggest mentioning the situation to the SC staff, and get their input. IIRC, I once saw a diver hire a Dive Master to "stay close and babysit me" (the divers own words) during the dive.That may be an option.
In any event, have a safe and enjoyable trip.
Edit to add: Hoag does a nice job of describing the dives, ( and types faster than
me!!) One thing: I last did the Shark feeding dive about 7 years ago, and it was as Hoag described. Since then I've been told by people that have done the dive more recently, that SC moved the location from the sea bottom to the deck of one of the wrecks. That may or may not matter.