On the Velocity of Light Signals in the Deep Underwater Neutrino   Experiments
  During the last few years deep underwater neutrino telescopes of a new generation with dimensions close to 100 m or more were taken into operation. For the correct track reconstruction and for the interpretation of light pulses from calibration lasers one has to use the group velocity for light signals. The difference between group velocity leads to an additional delay of about 10 ns for a distance of 100 m between light source and photjmultiplier. From the time of the appearance of the first projects of deep underwater neutrino telescopes in the middle of 70th this fact was never mentioned in the literature. 
L.A.Kuzmichev