Probably the most expensive method for finding water temperature you can think of. It uses an ultrasound reflection measurement and the dependence of the speed of sound with temperature.
The video is based on a project done at the Group for Digital Signal Processing and Image Analysis, http://www.ifi.uio.no/research/groups/dsb/, Department of Informatics, University of Oslo in the course INF3460 - Project in acoustics and signal processing,
http://www.uio.no/studier/emner/matnat/ifi/INF3460/index-... (less info)