The cheap device mentioned in the link below works as an underwater distance sensor, 50mm to 2000mm, so meets your 6 cm criteria, barely.
It is intended to measure tank water level through the tank bottom wall, but @wvmarle tested it by holding it in water, pointing it down, and it worked. He even wrote a library for it.
For your application, it may not provide measurements quickly enough. The link has the sample rate.