The following instructions will guide you through the worst submarine crisis situations. Please follow them carefully if your Sub is pinned down at the bottom of the ocean.
1. Do not, I repeat, DO NOT open windows or hatches to scream for help. Incoming water could ruin your shoes. If you do open anything, keep your mouth closed.
2. Use a heavy object to knock on solid surfaces like doors, frames or the floor. Do not knock on torpedoes. If you do knock on a torpedo, close your ears.
3. After not reporting to naval command for 2 hours, they will start a search for you and your 24 nukes. Do not panic, you have air for 5 days.
4. If you're in a Russian submarine, after not reporting to naval command for 5 days, they will start a search for your 24 nukes. Do not panic, you have air for 2 hours.
So, how do you actually call for help when you're pinned down at the bottom of the sea? Well, the rather disturbing answer is: you can't! When submerged, there are no possibilities to communicate with surface ships or naval command.
What means of communication Submarines do have? If they are on the surface, they can use radio equipment, just like any other ship. Although short wave HF radios can be used, modern subs use satellite communication to keep contact. With VLF, 3 to 30 Khz, the Subs can receive up to a depth of 20 meters (60 feet). If it is required to stay in deeper water, a buoy on a cable can be deployed, until a depth of a few meters. If you're in deeper water, or have no buoy, you have a problem.
Until 2004, the US subs could receive messages through ELF (Extreme Low Frequency) transmitters, ranging from 3 to 300 Hz. The transmitters use extremely large coastal underwater antenna's, 20 to 40 Km wide. You need a power plant to feed these monsters. Obviously, they can only send messages to you. You cannot send anything back, since you don't carry such large antenna's on your Sub. Due to the extreme long wave length, transmission rate is very slow. Only a few character per minute. The system was normally used to call the Sub to the surface, and continue communication with radio or satellite. Unfortunately, the US abandoned this system and the antenna's were dismantled in 2004.
So, Houston, we now have a problem...
More info on submarine communication can be found here.
By the way. The Russian mini-sub was strangled in a top secret submarine warning system antenna. Well, they can sleep on both ears, the warning system works extremely well. The whole world knows a sub was there.
Update: Frode pointed me to something totally new and interesting, SEADEEP blue and green laser.