Stealth
Noise
Though relatively quiet, your motors do generate some noise. If you're running at flank speed, that noise level will increase the range at which enemy sonar can detect you. Any lower throttle setting will not increase your detectability.
When below periscope depth, you can only detect other ships using passive sonar; the noise of your motors interferes with that, so the higher your throttle, the less "perception range" you have. You'll see the darker "fog of war" ring constrict as you increase your throttle.
The Thermocline
A Thermocline is a transition layer between water of differing temperatures. The thermocline is difficult for sonar to penetrate, so dive below it to make it harder for escorts to bounce a ping off of you. The blue marker on your depth gauge shows your sub's position relative to the thermocline layer:
