This is just Law of Ueki and Groundhog Day. It's absurd to believe that he kills himself so many times without psychological issues or negative personality changes. (All You Need is Kill does this better)
Also drowning is one of the worst and most painful ways to die. Please do not think that you just "fall asleep" the process is extremely violent because of how long you can actually survive underwater and the feeling of nasty river water filling your lungs before actually asphyxiating is something I do not wish onto anyone.