One of the things you learn in scuba is what to do if you run out of air underwater. There's five different things that you can do, and they all come in a priority order, based on your personal safety and that of your buddy's.
The first is a normal ascent. This is what's done when you're really low on air, but still have enough to make it to the surface safely.
The second is alternate air source. That means you breathe from a pony bottle (a little bottle of compressed air about the size of a waterbottle that will give you enough to reach the surface), or you breathe off the second emergency regulator ("octopus") on your buddy's tank, and head up together.
The third is a controlled emergency swimming ascent. That's when you have a lungful of air, and you slowly swim to the surface (1 foot per second, which is always the ascent rule), exhaling as you go. This works because as you ascend, water pressure decreases and the air in your lungs expands, so you can exhale for a long time on that single breath.
The fourth is buddy breathing. This means you and your buddy share a regulator by passing one back and forth, each taking two breaths as you ascend. This is ranked lower because you have the regulator out of your mouth and because you're putting someone else at risk.
The last is a buoyant emergency swimming ascent, which is what most panicky scuba divers will do. You basically hold your breath and try to reach the surface as soon as you can. If you hold your breath, you can get horrible lung overexpansion injuries and die. If you go to the surface too quickly, you can get decompression sickness (bends) and be paralyzed or die. Obviously, you don't ever, EVER want to do this.
In scuba, we practiced all of these ascents except the last. I always did fine on all of them except for one, the controlled emergency swimming ascent.
I only ever did it once successfully...in the pool, and it was my first try. You take a full breath of air, and then exhale very slowly as you ascend. It helps if you say "eeeee" very softly, because that released a very little amount of air, but keeps your air passages open so your lungs don't burst. Here's the catch...you have do do this from a depth of about 20-25 feet. That means that since you ascend at 1 foot per second, you have to constantly exhale for about 20-25 seconds...maybe 30 just for safety. Also, your air doesn't compress all that much at that depth, so there really isn't much significant expansion as you rise. Bottom line: I was running out of air.
We did this on the second day of the open water dive, on the first dive of the day. We went under, swam out to about 23 feet, and then Dan took us aside to do this. I tried it four times and failed every single time, and every time I tried it was harder because I was getting more and more out of breath and more frustrated. I'd get up to where I was about 8 feet from the surface and just be dying for air and have to suck in on my regulator (we keep it in the mouth for safety, since it isn't really an emergency and it's stupid to endanger yourself). Every. Single. Time.
Eventually Dan just took me back down and we finished the dive, and he said we could try it again on dive #4, the next (and last) dive of the day.
I practiced on land, and then when we went back down, EXACTLY the same thing happened. I was honestly in tears by the end of it. It was my eighth time I'd tried and failed at this, and this skill is absolutely necessary for certification. If I didn't do it, I wasn't getting certified...but I simply could not do it. I tried, and tried, and no matter how big a breath I took, or how slowly I exhaled, I kept running out of air. I should have had enough air that some would be left over at the end, but I didn't.
I was in misery at the end and I just went back to shore without even caring about the exploration part of the dive. The divemaster that escorted me tried to cheer me up by telling me that I was lucky, because now I could go and explore on the Coeur 'd Alene certification dive next month and try again (Dan said he wouldn't charge me for any of it, since I'd already paid for this dive). But the thing is, I didn't WANT to go to Coeur d'Alene. I still don't. When I was diving in Seattle, the water was so cold that we had to wear thick wetsuits and constricting hoods that hurt, and our faces ached with pain at the end of the dive from the freezing water. In April, Lake Coeur d'Alene will be exactly the same temperature. I will have to drive all the way up there and back, which is about 2 hours each way. I will have to rent fins and gloves again. I will have to waste several hours of putting on equipment, adjusting weights, sitting around, then diving and sitting around, being absolutely freezing cold. I spent that entire weekend shivering uncontrollably and basically having very little ability to use my fingers at all. In Seattle, there was a little bit of stuff to see on the bottom (lots of crabs, starfish, sea anenomes, sea cucumbers), but none of that exists in Coeur d'Alene...I think that it's mostly worms and garbage. All I can really hope for is that the visibility is better. The bottom of Puget Sound is so soft that our visibility was about 3 feet...and believe me, that is NOT much. I was starting to feel claustrophobic...and I'm not claustrophobic at all.
Tyler's dad said that your certification dives are like boot camp. You have to do them, and it's usually in a place you don't like, wearing gear that doesn't fit you and doesn't work. That's about how I felt. After I get certified, I don't have to dive where I don't want to, when I don't want to...but at least I have the chance and the ability. I have to get certified. If I get left out of a dive, it will be because I chose to stay, not because I wasn't able to go.
So I had to go back home, explain to my parents, and then deal with the rest of my Spring Break being utterly depressed that I'd gone through all that work and trouble and was going to have to do it over again. I wanted to crawl under my bed and die. I was depressed that because I couldn't do a simple 30-second exercise, I was going to have to waste a day in Coeur 'd Alene on a weekend before I have a crapload of work due. I was depressed that I tried and tried and tried to do a very simple thing and that my body simply wouldn't let me finish ("BREATHE!"). I was depressed because I was sandy, sick from salt, utterly exhausted from being inside a stiff wetsuit and carrying around heavy equipment, and my horrible wetsuit that my divemaster lent me smelled like a zoo. And it cast a miserable pall over my whole Spring Break.