Anti-echoic tiling is a spongy rubber material that covers the
hull of modern
submarines. This sound dampening coating means that
active sonar will not reflect off the hull as easily so a searching boat or ship must be closer with a more powerful sonar to detect any sub equiped with the tile. The tile cladding also reduces the amount of noise leaving the boat, making it harder to detect on
passive sonar or
hydrophones.
The primary sense for the marine world is sound, so the goal in creating a stealthy submarine is to keep it quiet. In some situations the sub will shut every non-essential system off to reduce noise. This means ventilation, laundry, galley, and showers. This is what submariners mean when they say "Make like a hole in the water."
See also: isolation raft for more information on submarine quieting technology.