I'm thinking either red or lime green for day, and SOLAS reflective tape (maybe with that glow-in-the-dark stuff) for night.

In the few times I've been offshore, I have a few items in my camelback pouch:
- dye marker
- whistle
- handheld strobe
- signal flare (sold as PWC safety kit)
- handheld (submersible but in waterproof bag, too) vhf

Cell phones don't usually get reception beyond 4-5 miles offshore.

All that stuff probably weighs about 2 lbs.