This is something I've considered but never tried, possibly with diazepam as I know what effect a strong dose has on me. Maybe it's a romantic notion, but I'd like to die like that, in a lake surrounded by beautiful scenery. And the people searching for my body would be experienced in that field (search and rescue) rather than someone happening upon my body. And I figure the cold water would help with hypothermia even if I didn't actually drown but just dozed off (done that before. I find water extremely soothing).
However, when I look at closely, it all sounds rather unreliable so I reshelve it as a nice idea and carry on.