From my perspective, it was a complete freak set of waves. Essentially, we were tied off to the Black Bart, near the stern. The line descent was probably around 100'. Our group completed the dive in about 25 or so minutes. Another couple did not dive because of equipment malfunction. We boarded the boat, me first, then my other buddies. Procedure called for us to move towards the center of the boat to disengage our BC and store it down on the deck. I took off my BC, my gloves and moved my fins to be near my bc. Ohonu, was the last to board and did exactly what we did. One of my buddies and I moved to the bow of the boat to redistribute the weight. The other couple also moved to the center of the boat. Once Ohonu was on board, the captain signaled the DM that it was time to untie. The DM splashed and descended down to the Black Bart. The captain had started the engines and moved towards the front the boat to assist with the tiein line. At this point, Ohonu was near the rear the boat when the first wave splashed, putting about 4-5 inches of water into the stern area - Ohonu calling me to get my gear. He barely finished that sentence when the second wave crashed into stern essentially pushing the stern down, burying 3-4ft of water in the boat and the engines quit - seconds later the third wave swamped the rear, alarms went off and the captain began the mayday sequence. Meanwhile, the DM had untied the boat from the Black Bart, so we were essentially free. The captain was able to relief the line so the DM would not be sucked down if he was ascending with or near the line. From the time the first wave hit until we jumped ship was at most 45-60 seconds. The captain did get the life jackets out to everyone he could before the boat completely stood up and rotated deck down. It was at this point that I jumped clear and something flew off the bow of the boat during the rotation and hit me in the head. As the boat completed its rotation and settled hull up and pointing like the titanic, we gathered what we could to float and it was then that they noticed my injury. The captain immediately swam over to me, put pressure on the cut and we collected some flotation seats to keep me afloat, in addition to the life vest. We took stock of the equipment we had on us, some fins, the DM had his gear and life bag and whistle, rope, etc. Luckily, a can of Sunkist floated by...nothing like a cold Sunkist on a hot day.
The captain used that to keep me hydrated. From the DM splash to his surfacing could not have been more than 2-3 minutes, TOPS. I'm sure he was a bit surprised to see the boat floating upside down. All in all, everyone kept their heads and we made it out ok. Again, KUDOS to the captain and DM.