In my very limited experience, there aren't any caves or caverns around Oahu that are anywhere comparable to what's in the upper 48. We do have lots of lava tubes but sea action tends to collapse or unroof them. So most of the stuff here tends to be fairly straightforward swim-thrus; e.g. Makaha Caverns and Shark's Cove and outside of Malaekahana Bay.
I've been diving Shark's Cove since the mid -60's. But although I might have more general memories of the place, ScubaDrew's undoubtedly got the place wired in detail far better'n me. His truck's been at Shark's Cove at least as often as I have this summer -- well, I haven't seen it weekdays.
Sorry, I also don't blow bubbles anymore. But I'm nowhere near Deaf Paul's league -- never will be. My limited abilities are adequate for Shark's Cove.
At Shark's Cove, what's well-known as the "Elevator" is what we used to call the "toilet bowl"
old post No kidding, more than a few people have gotten trapped and died in there; the 3 Marines who got eaten are mentioned here
1987. It's practically irreversible; once in, you're committed. Don't try it if there's any surge pumping. Tighten down and streamline your gear; especially have control of your BC cross-section, tank, and any danglies. Have negative buoyancy. If you have a hood, wear it to cushion your head. Make sure it's clear before entering; do not go in on the heels of another; if she gets stuck, you both might be screwed. I prefer to extend one arm forward with the other tucked down near my chest -- the extended one will pull along, shine the light, and clear any surprise hazards (urchins, eels, and a long time ago, lobsters) while the other is in position to get control of any loosened or problem gear. The tunnel is mostly rounded rocks and fairly straight. It can silt up bad but it's a simple tube; just keep truckin' forward.
Except for the cavern at the far right-side point, most of the other tubes boring into the shelf are pretty much dead-ends; pinching down to impassable. For these, because there's no room to turn, my technique was to go in feet first. That avoids the danger of becoming trapped from going in headfirst and, while trying to backout, the BC or tank bottom hangs up on projecting rocks. Besides, if there's something already in the tunnel, I'd rather have it take first whack/bite at my fins than my face.
As for Ulua Cave, item #3
here
