I've learned with number three, that red and green glowsticks are a Godsend! I always carry a few strapped to my pony bottles and some strapped to my BCD --- bend and shake and you've got light.
You usually get them in Green, but bright white and Xenon Blue are available now!
If you're in a wreck or a cave you can also buy 2cm diameter weighted clear plastic balls which contain a single blinking white LED and some watch-style batteries which can last up to 12 hours even at depths past 230 feet (70m). They usually have a loop with some string and a largish clip which you can put on some coral, kelp or hull to keep in place along with it's weight.
On the way back out you pick them back up to ensure your safe passage the way you came in!