[MARMIR]

...Come on folks let's be serious, this is the usually lie that we say to ourself to make us feel a little bit more confortable when we start going down under....unless your buddy is up in a boat holding a rope tied to you or watching what you're doing I wouldn't rely much in a buddy who is spearing like you......if something goes wrong he will rescue at least 15 minutes later (if you are lucky)
I would rather rely on a simple device: a kind of life vest with a small computer that would shoot me up to the surface once a preset time is expired.....