A couple of things to know:
Some of the charters don't run in the winter at all. The only one of the three mentioned above that
I know does is Express Watersports. The water will be in the low 60's, and weather is always a factor.
Most (all?) of the diving is off boats. Mostly man-made objects except for the Ledges. What makes Myrtle Beach such a good beach makes for lousy shore diving, it's very flat with a sand bottom for a long way out. So just about every dive is a long boat ride.
We dove the Perry Wreck, your g/f could probably snorkel it. The bottom under the boat was just about 60'. Vis was really bad though. Another shallower wreck is the Tugboat/Airplane, they're at 40'. At both sites, it was just them surrounded by flat sand farther than we could see. So be aware that you're not going to see a lot of stuff. Also they spearfish the wrecks since the fish congregate there.
Express has divesite listings/depths on their site
here. Notice that on their map, every site is some distance offshore.
We looked for shorediving in the area but weren't able to find any sites. Talking to Express, they told us about a possible site on a breakwater of some sort but it would've been a long trek from the road with gear. Might be more doable snorkeling.
Since your g/f likes Sharks, maybe drive to NC and do the wrecks. Sand Tigers live there. I don't know if they run in the winter either as I understand that some of them are long rough trips even in summer.