I've only tried shore diving a couple of times on Oahu so I'm sure there are many places I don't know about. I tried around Waikiki (found nothing), at Haunauma Bay (a few miles east of Honolulu), around on the eastern shore (also didn't think it worthwhile), and a couple places along the north shore (very dependent upon north swell at that time of year). Sorry but it's been so long I don't recall the names of the sites I tried.
Haunauma Bay is a fabulous place - amazing coral and very protected from ocean conditions. It's a popular beach spot though and pretty regulated, so best to get there early, and check about the current parking or access conditions beforehand. The shallow area right next to the beach is thrashed from all the tourists, but if you swim out past the shallow reef top that acts like a breakwater, the good stuff starts. It's not a far swim at all, and then mostly shallow as I recall, 30 ft or less. I haven't been there in 15 years, but it was awesome and it would be near the top of my list if I got back to Oahu. I probably won't though because I think diving the Big Island, or Maui (distant 2nd place, if it's shore diving we're talking about), or even Kauai, is better.
If your clients are only looking for a quick day or two of diving, they can probably be pleased with any of the islands, including Oahu. If they are reasonably capable at shore diving, I'd highly recommend the Kailua-Kona area of the Big Island, where there are literally dozens of good to great spots.
