If you've got something other than a NAVI, I'd:
1) Turn the unit off and back on again when swapping sailors. This should make a new file, or at least make it easy to see where your sessions start and end.
2) Write down the time each sailor had the unit.
3) When a sailor gets the unit, that sailor should walk around in a certain pattern before sailing. So sailor 1 might walk round in a circle, sailor 2 might go in a triangle, sailor 3 might go in a cross pattern. When you look at the data you'll see a shape appear in the track so you know who's got the unit