Sorry - been on a business trip. Let me review some clarifications from the above posts.
Yes, it was the O-ring between the tank and valve. Yes, it was a boat-owned tank as I had requested EANx and was required to use a boat-owned tank. Yes, I notified the group leader before we left the dock that my tank was near empty (to tell you the truth - at that time I too thought that the tank wasn't filled), the boat DM the next morning that it was down again in spite of not having used the tank nor having my reg on. They both hunted for, but did not find, another O-ring.
Later, before dive 3, I was really miffed. As everyone else had already gotten in, my buddy and I were on the deck and I refused to enter the water with less than 1500psi. The captain came over and asked if we were skipping the dive. I told him what was up and at least he topped me off so that I could at least start the dive with a full, but leaking tank.
And finally, yes, this is why I won't ever get on that boat again. I talked with Frank and assured him that I'd dive with him again. Frank and Melanie run a class operation. I'll never know about the other boat though.
Of course I was shocked. As you can tell from my profile, I am not a new diver and I've been on my share of trips. As a matter of fact I was already an experienced instructor (PADI MI) when this occurred. And I normally always carry a full compliment of O-rings in my repair kit, but didn't bring them this time - just the usual ones for the yoke. Why? I don't know, but I didn't worry about it because I knew that I was using a provided tank.
I was stunned more by the lack of concern over this situation than the event itself. Stuff happens. And I did email the company (they were still a unified operation then) but didn't even recieve an apology let alone any credit, etc. I know that it isn't a life's savings spent on the trip, but when you're working 7-day weeks for months on end and finally get a weekend off to dive, this wasn't my idea of how to spend it.
As a matter of fact, after my email Frank replied to inquire into some details but I never heard squat from the other boat's captain or the then-owner.
Bottom line: I'll dive with Frank and Melanie any time. They have a great operation. And besides it is fun to watch Melanie get spun up when a diver does something stupid. But that's it.