[tjpnz]

Neutrino interactions with nuclei of water produce charged particles which move faster than the speed of light in water (but slower than through a vacuum tube) creating a cone of light known as Cherenkov radiation, which is the optical equivalent to a sonic boom. This is projected onto a ring around the detector.

Doesn't need to be submerged in a body of water as large as this. The Super-Kamiokande[0] detector for instance is located in a body of water inside a mountain.

0: https://en.m.wikipedia.org/wiki/Super-Kamiokande