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Radar  system uses electromagnetic pulse and its  reflections to determine the position of the object. The  Sea water conducts electricity  well and  absorb the electromagnetic energy of the radar by not allowing  it to penetrate  to a significant depth.So radar cannot be effectively applied under water.

Consider the case of SONAR , water is good for propagating sound will although it propagates much slower the RF waves, the distance that sound waves travel is  longer than radio waves   in the water. SONAR is used for echo location underwater due to this reason but it is much  slower than the radar operation when compared.