Hydrophobia

The best guide on the internet for these medals can be found here:

http://www.xbox360achievements.org/forum/showthread.php?t=29...
Credit to StarsRPD

It's worth mentioning that some medals may disappear between playthroughs, therefore it's recommended to try to obtain as many during one playthrough, followed by a run through the challenge room.

If you have accidentally screwed up the Deep Breath medal by scanning the wall and not getting the medal, there's a way to get the medal without starting over completely from scratch.

There are individual save files for each act. So I got my character to Act 3 with 46/47 medals. Then I moved all my save files (including options) to my cloud storage and deleted them off my hard drive. Then I started a brand new game on my Hard Drive and did a speed run to Act 2 on Easy. Once in Act 2 I got the Deep Breath medal and then proceeded far enough in the level to get a check point.

After doing this, I quit out from the game and went back to the storage options on the Dashboard. I then moved my Act 3 save from my Cloud storage back to my Hard Drive. I reloaded the game, continued the game (in Act 2) to make the game think I had the Deep Breath medal. Then I exited the game (to the game menu, not the dashboard) and started a new game from Act 3. The game then registered I had 47/47 medals but didn't give me the achievement. I proceeded into the level, killed the first enemy I found, looted his corpse, and I got the achievement.

Thanks to DarkRedScorpion over at Xbox360Achievements for the idea.

One thing more to mention in addition to the xbox360achievement guide that gave me a free card to another playthrough.

You have to successfully hack ALL devices you come across and have to play through the game with ONLY using your Sonic Rounds. They are lethal after the patch, so there is not much problem with this one. The guide tells you this at the end ...

The by Caduceus mentioned Deep Breath Medal in the beginning of act2 gave me troubles too. But its really easy if you know how to do it. At the beginning, turn around inside the elevator - dive down, take a right, collect both documents from the corpse (2 times X), turn left, scan the wall and speed back.

Another troublesome one was the Flood Medal in the beginning of Act3. To achieve this on - open the first door with the Camera (it stays open this way), destroy the 2 glass walls in this corridor, in the next room open the door you came from with the camea to flood this one. Dive to the opposite door and in the next corridor destroy the glass to your right and the 2 in the right corridor - NOT the one left.

For some medals where you need to kill someone (Stealth Medal in Act 2 where you need the gel rounds and Humanitarian - just reload checkpoint after achieving the medal - it will not disapear and you can kill the guys with Sonic amunition).
If you see you will fail at a hack, just hit B and try again, it will not count as a failure.

Hope this made some points clearer and spares you a second playthrough.

As a side note, if you are having trouble getting the Deep Breath medal, (as the way the guide stipulates confused me at first), there are some steps to take. If you consistently repeat the requirements to get the medal and it does not appear, go to the Dashboard. From there delete your saves, including the save that says "Options." This will delete your medals, documents and objects. Reload your game and speed-run to the point in the elevator shaft.

Go to the top of the shaft and then swim down. Grab the two docs AND scan the Mavi symbol which will now be able to be scanned. As long as you do this and resurface in a breath, the medal should appear.