I could help. Do you know your exact coordinates? x/y/z coordinates? Or your depth? I can at least check the condition of that tunnel to see what might be the matter. I'll head there now and poke around.
Edit: Okay, I left two chests with waterbreathing potions, sugar cane, and torches. If you're super low on health I can get you some golden apples if need be, but I didn't think about that until after I had already dropped my supply crates. So here's some things to guide you:
The orange ellipsis is where I think you likely are- the envelope around the tunnel that has lighting in it. That's the only enclosed place, so if you're not there, I suspect you would just swim straight up no problem. I didn't see any problems with your tunnel flooding. The second screenshot is a photo of me standing in the middle of your tunnel envelope, the highest lit point (aka, a glowstone). I placed one chest 1/4th the way from the bottom, and the other chest 3/4ths the way from the bottom. They're dead center given the width of the tunnel, and are on the sea floor. The third screen shot is facing your tunnel from the ridge to the east of it (underwater). It is possible you are behind that area given your circle, but I doubt you're outside the envelope anywhere to the west of that because it is too shallow.
http://i.imgur.com/hgJROGT.png
http://i.imgur.com/wKGh87V.png
http://i.imgur.com/Nlp8qcc.png
If you don't believe you're in the envelope of your tunnel that is filled with water, I'm going to need a much more exact location (open the game, press f3, then f2, exit out, find the screenshot in your folder), because I didn't see anywhere else obvious that you could be. Also, if you're super duper low on health/breath, I could also just pick up your stuff off your corpse if you don't think you could make it. Last resort obviously, but... I'm in the neighborhood. If my two chests aren't close enough, I've got a third water breathing potion and chest I can drop closer to you if I know where to put it.