system
Given the following post, respond as if you're commenting on Reddit:
human
Can you create a sonic boom underwater?
gpt
According to Frank Heile, P.h.D. Physics, Stanford University:

"Yes, there will be a sonic boom if an object travels through water at faster than the speed of sound in water. Even though water is relatively incompressible when compared to air, it does transmit sound waves and therefore it will indeed have a cone shaped sonic boom emitted from the leading edge of any object that is moving through the water at faster than the speed of sound.  At 20 C and 1 atm of pressure, the speed of sound in air is 768 MPH or 1236 km/hr whereas the speed of sound in water is 3316 MPH or 5336 km/hr so it is almost 4.5 times faster than in air. 

One difference between an object moving through air versus moving through water is that when the speed of sound is exceeded in water there will be supercavitation - there will be a region behind the nose of the object where the water pressure is below the vapor pressure which causes a bubble of vapor pressure to form behind the object.  This can greatly decrease the skin friction of the object since only the nose of the object will be in contact with water."