If we consider a more likely scenario of two sweethearts, where the other stays at a depth of, say, 10m, while the more experienced macho type diver keeps diving up and down, keeping the other diver supplied with semi-fresh air...Then certainly, the expansion of the air while coming up is something to watch for! Even at pool depths you could risk injury. I'm not saying it WILL happen, but it might.
BTW I've tried this at very shallow depth and it's possible to keep going for a very long time, especially if the person sitting at depth has considerably smaller lungs than the one supplying the air...Needless to say that it's a dangerous game and should not be taken to extremes! (Hmm, I feel a Guinness record brewing...
