Yes, you can dive it as well described be vbluev and zackss. But it sounds like you're thinking solo and if so, I second selo's concern -- I almost got taken out. It was a nav error compounded by surge and surf; I missed the "Backdoor" exit. I got shoved and pinned by the following surf into a reef depression just deep enough for the waves to continuously wash over my snorkel. My arms got pinned at my sides so that I couldn't recover my reg or even simply pushup to catch a breath. No choice but to hold my breath and wait for the set to finish rolling in, then logroll just enough to free my arms.
It's fairly shallow (15-30ft) until about midway out the Bay. The surge and clarity might not be what you like until you get farther out. I haven't been very far beyond Witche's Brew but I heard it's really nice.
There's a way to gauge the outgoing current strength at the Slot. Looking out to sea, there's a general counter clockwise current with the water incoming through the Backdoor and exiting through the cable channel/Slot. Basically, if you look down from the access road, Backdoor is marked with buoys on the left, the Slot is marked with the buoys to the right. Most days you'll see the surf breaking bigger over the Backdoor than it does at the Slot.
Again looking down from the road, you'll notice deeper blue spot inside the reef, just to the right of the Slot. This is Sandman's Patch; about 10ft deep. There's a large rock formation on the seaward side that you can swim all around. It's actually a bit hollow with passages to look through; loaded with soldierfish Look for a cleaner wrasse station on the right side. Here is an opening to the underside of the formation. With luck, you may catch a glimpse of a whitetip reef shark napping. Poke around the right side border of Sandman's; there's another hole where a whitetip sometimes spends the day.
As for the main swimming area, one challenge is to find panther flounders. Skimming the bottom is the easiest technique.
p.s. The tsunami pushed the buoys about 5m shoreward. On Sunday, one of the local bruddahs (not sure if he was staff) spent some time diving down and humping the huge anchor stones back into position -- they do that kind of thing anyway as conditioning for big surf. There was still some small rubbish floating in the water and along the high water mark which was about to the grassy areas.