I coach a girl who does long-distance open-water swimming - like the Channel, that sort of thing.
We think that some sort of GPS or at a minimum a compass would be a good tool for her to wear, both while training and while actually doing the swims.
One of the things we'd like would be some sort of feedback on direction. There are a number of issues/characteristics with this:
Aural feedback may not work well. Bone conduction is a possibility, but earphones probably won't stay put for the length of time she will need the info.
An led indication of course could work; maybe a tri-colour attached to one goggle (green ok, red veering left, yellow veering right, that sort of thing).
There is a bit of body-motion to consider. She could wear a belt with the unit in the small of the back, or on the head if it is not too large. The back would have mainly roll; the head may have a bit of yaw as well, so that might be trouble.
Initial setting of the course would have to be worked out - if the unit were on the head, perhaps she could orient as desired, then press a button to fix that course.
Course indications would need to be integrated over a short (~5 to 10 sec) period. It might be helpful if this period could be randomly varied within those limits, as some swims do not allow "pacing", and a repeating signal could be used for this - unless it were irregular.
For training she often does multi-leg "loops", returning to a starting point several times. For that some sort of GPS-type distance logging, course & waypoint setting, etcetera would be a plus. This would also be a BIG plus for some swims where current is a variable.
Waterproofing needs to be enough to withstand shallow immersion - say 30 feet.
Battery life might be a factor, some swims can be more than 10 hours.
Radio coms would also be a plus, but rather than get just a radio and have a guide give info, a self-contained unit would be much better.
I can do small electronics, uc programming, and machining, but have very little knowlege of what building blocks are out there and their qualities - like how fine a resolution can you get from electronic compasses, what tricks can you get GPS to do, will these blocks work if under 2 feet of seawater, current draws etcetera...