What sense organ is indispensable in troubled waters?

Dolphins (and some other marine mammals) have a sonar, and fish have a lateral line.

The lateral line with receptors located on it in fish catches water vibrations and gives signals of a possible danger, an obstacle to the brain. Surprisingly, this orientation system is indeed very sensitive and can completely replace vision at night or in troubled waters. Sonar, as the name implies, picks up echoes from sound signals sent by marine mammals.