In addition to the other answers already posted, the neutrino may hit multiple water molecules along its path, or its decay products may hit other molecules themselves, so you get many flashes if you're lucky.
But another category of detector [1] adds additional signal by applying a strong, constant electric field vertically across the entire detection chamber (heavy noble gas, not ice, in this case). Then whatever charged particles are produced drift up to the top of the tank, are annihilated there, and you get a flash that gives you extra good localization in the z direction since you know how long it took them to get there.
But another category of detector [1] adds additional signal by applying a strong, constant electric field vertically across the entire detection chamber (heavy noble gas, not ice, in this case). Then whatever charged particles are produced drift up to the top of the tank, are annihilated there, and you get a flash that gives you extra good localization in the z direction since you know how long it took them to get there.
[1] https://en.m.wikipedia.org/wiki/Time_projection_chamber