A while back I downloaded an Android application called "AR First Aid."
It was pretty good, not that I ever needed it for a real emergency, but I tried it on a lot of hypothetical problems.
What caught my attention, though, is that it is regularly updated and seems to be getting better all the time.
I haven't tried it this way, but it seems that if you update before leaving range, that you can use most of the features while outside of cell range.
The bad news is that it is still dependent on an electrical device that will fail immediately in salt water.
For what it's worth...
: San Diego