I like the idea of using a bag sealer and sealing a Striker and Tinder. Make sure that when you test it you go below 100' so that it gets fully compacted by being exposed to 4 Atmospheres. this way hen you open it (after you get to shore) you can ensure it will function. Try a few kinds of tinder and see which light better after compaction. Wetfire should be unaffected, but since I never carried any firelighting gear when I was a scuba instructor I am just guessing.

Neat idea by the way, hope you test programme goes well.