It just became possible, thanks to a laser camera developed by physicists at Heriot Watt University in Edinburgh, UK. You can pretty much see how it works from the images above -- laser beams are bounced off the target and the duration of their return journey is measured precisely enough to detect millimeter depth changes even at extreme distances.
The floatable bags operate in extreme temperatures of -22F to 140F and protect against water submersion for 30 minutes at a depth of 1 meter, also designed with room to accommodate a few small items such as credit cards, bandages, etc.