To add my two cents, I had a similar experience - on my first open water "checkout" as you americans seem to call them (hadn't heard that term before) I suffered a panic attack/anxiety attack.
I'd been having trouble with my buoyancy, which I know now was because I was taking big, deep, huge lungfuls of air, then one of my fins came loose and in my panic to secure it (when you've just shelled out $150 for a pair of fins, its not good to loose one on your first dive in the ocean!) I overexerted myself, got a cramp in my calf, couldn't breathe, had to surface.
I dragged my buddy up with me, who was a bit confused - "Whats wrong?" and thankfully one of the instructors surfaced as well. As it was near the end of the dive, he secured my fin, removed my cramp, calmed me down, descended and got the rest of the group to ascend while I remained on the surface, then towed me back to shore.
I was so exhausted when we got to the shore that I couldn't fathom how I was going to get back to the dive shop, let alone back in the water. I was diving in a 7mm wetty with long johns and jacket = 14mm on chest, so I had like 14kg of weight on. I couldn't stand up at the end of that first dive and the rest of the weekend was streching ahead of me. I didn't know how I was going to continue.
Thankfully, the instructor sent everyone back and sat with me on the beach while I got some energy back. He talked me through what he had observed happening and we worked out a plan of attack to overcome my breathing problems. When I felt recovered, we walked back to the shop.
We had two instructors for our class and before that dive, I'd found this instructor stand-offish and felt like he thought he was better than us. Looking back on it, I seriously doubt I would have finished the course if he hadn't spent that extra time with me.
I learned to dive in what I feel are harsh conditions - cold, salt water on hot days (typical Melbourne summer, the water never heats up) and for this I am glad. My sister (been diving for years) told me that if I could dive in Port Phillip Bay, I could dive anywhere. I'm so glad I learned to dive in those conditions, as opposed to in tropical where I have since dived with just a lycra suit and like 6kg of weights. I feel it would have been very difficult to go the other way - from tropical to Melbourne!
Since my OW, I've clocked up 20 dives (not many I know!) and have just completed my Adv. I'm now looking to the future - instructor here I come!
Keep at it, shaken_babe!