I don't quite follow what you're trying to do, but I can share my experiences. When I dive locally, I sometimes carry a small GPS receiver (Qstarz BT-Q1000XT) in a small plastic case (Pelican 1010) connected to a line, which I hold or attach to myself. The case is waterproof and bobs on the surface above me while I swim along the bottom. When the dive is over, the GPS track can be merged with the photos to yield GPS-tagged underwater photos. Presumably one can replace the altitude information in the GPS track with depth data from a dive computer, but I haven't tried.
This arrangement has some practical problems/irritations. The GPS track is intermittent, although if I find something interesting underwater, I'll hang around long enough that it will record some points. Having to manage a line to the surface while handling an SLR with strobes can be a bother. An alternative would be to carry the GPS and case in a BCD pocket and only deploy them to the surface if something interesting turns up, but I don't trust the Pelican case at depth.