Since extremely-low-frequency radio waves are used by submarines for some simple, low-transmission-rate communications, why can't those same wavelengths be used for submarine radar? It may not be ideal, or highly precise, but couldn't it still provide some useful information?
There are likely many reasons why ELF radar is impractical. Here are two. The wavelength at ELF frequencies exceeds 10,000 km. A radar using that large a wavelength will not be able to locate anything very precisely. In addition, radiating and receiving radio signals efficiently requires an antenna size of a significant fraction of a wavelength; probably too big to fit on a submrarine.