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The problem with inertial units, without gps, is no way to effectively account for ocean currents..which could mean hundreds of yards, to miles in just an hour long dive, depending on where you are diving.
I don't think that's the case. What inertial units do is doubly integrate the instantaneous acceleration (all they can sense) over time to derive velocity and then distance. Newton's laws of motion apply: d = 1/2 * a * t**2, simplifying for 0 initial distance and velocity. Currents may be substantial, but they can't move the diver in space without accelerating him at some point, even if that's one big pulse - or velocity step function - when he jumps in the water. The unit will sense that initial acceleration caused by the current, and any subsequent changes, and that ought to be enough. It doesn't matter what causes the acceleration, and there is no motion not caused by acceleration.
The problems are that the tiny errors or noise in the acceleration sensing of reasonably available real-world sensors, integrated twice to calculate velocity and then distance, over an hour or so, result in unacceptable final errors in calculated distance. Other complexities and sources of error are that the problem needs to be solved in three dimensions (or maybe 2.5 for the diver problem), with instantaneous orientation of the acceleration sensors also an unknown that must be derived from that same acceleration data, perhaps aided by an electronic compass, depth sensor, or whatever other cleverness can be thought of - but those compass etc. sensors also have errors and noise. The report of the study emoreira pointed to a few posts above is some pretty good reading on this.
[The above wouldn't be the case for the 'pedometer' approach, like the Honeywell Dead Reckoning unit, one of several reasons why that doesn't translate very well to diving, e,g, sensing fin strokes. That doesn't work on land, either, if the unit rides in a car].
And then there is always the Radar reflective Flag, that any diver could hold up with a speargun or on a good safety sausage--all the boat would need is simple radar and a Garmin chart plotter..
It takes quite a flag to actually be effective. See http://www.scubaboard.com/forums/re...3968-radar-reflective-smb-safety-sausage.html