Amy, if you haven't been to Sunset Beach before, you're in for a treat. IMHO, it's one of the best spots on the NC coast!
Your closest options are Coastal Scuba in Little River:
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Coastal uses a marina in Little River that is about a 15-20 minute drive from Sunset. Their boat is a little rough around the edges, but the crew is friendly and helpful, I don't know what the capacity of the boat is, but I would imagine it's less than twenty.
And Scuba South in Southport:
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I haven't been out with Scuba South (we're going this weekend to the City of Houston with them). They are located in Southport, about a 35-40 minute drive from Sunset. Southport is worth a side trip in an of itself, if you like little fishing villages.
Probably your best bet is Aquatic Safaris in Wilmington:
Aquatic Safaris, North Carolina Wreck Diving
They run two different size boats, plan on about an hour's drive from Sunset to be safe, but it will probably take less than that.
If a longer drive (hour and a half plus) doesn't put you off, there is Scuba Express in Murrell's Inlet, south of Myrtle Beach, they're another quality outfit with a newish (but larger) boat. They took very good care of us when we went out with them to the City of Richmond. Their website is:
Express Water Sports - Scuba Dive, Parasail, Kayak & Ocean Sightseeing Dolphin Watch And jet skis in Myrtle Beach South Carolina - Home
Hope this helps,