Good questions.
I'd avoid a dry bag, except for use on water -- too heavy for most apps. Simpler to put a trash bag inside your pack, then to put items into ziploc(tm) or similar small bags. Eletronics*, paperwork, bandages and tinder could go into dry boxes ot tupperware(tm).

Avoid trouble and buy waterproof flashlights and watches.

Teacher

( *cell phone, gps, radios, etc.)