Use a good plastic bag to line the pack and it will float very well if you close it well enough. Pack everything in plastic and your bag will float almost too well.

All of the basic necessities are the same, shelter, water, warmth, light, signaling, etc.

The desalinator is a good idea, but they are expensive.

One primary problem from most of the latest incidents is sanitation. Hand sanitizer, wipes, etc.

Satellite phone is also a good idea.