Why radio doesn't work underwater
Salt water absorbs radio waves within metres. A 100 MHz signal that travels 200 km in air dies in 1 metre of seawater. Sound is the opposite — air absorbs sound quickly, water carries it for hundreds of kilometres. The choice is forced by physics: above water, radar; below water, sonar.
Speeds and resolution
Radio travels at 300,000 km/s. Sound in seawater travels at about 1.5 km/s — 200,000 times slower. That means sonar has 200,000 times more time per pulse to resolve range, but a 1 km range fix that radar gets in 6 microseconds takes sonar over a second. Sonar trades latency for resolution.
Active vs passive
Active sonar sends a 'ping' and listens for the echo — analogous to radar. Passive sonar just listens, identifying ships by the unique acoustic signature of their propellers and machinery. There is no passive radar in the same sense, because targets don't naturally emit detectable radio waves (with one exception — passive radar can use ambient TV/cellular signals as illuminators).
Display conventions
Both systems borrowed the green-on-black PPI display from WW2 radar — sonar adopted it after 1945 because submariners trained on radar were already familiar with it. The waterfall display, where time runs down the screen and frequency runs across, came from sonar and migrated back to electronic-warfare radar.