Just a note about keeping old flares and the 12 ga. flare gun shells as spares.
Unlike shotgun shells and metallic cartridge ammunition, they really do age quickly and don't work reliably even if they look like new. I removed the oldest stuff from my boat and my sons boat, took them home and we tried to set it off during 4th of July.
Only two of eight flare gun shells worked remotely close to design, the rest fell out of the barrel, smoldered on the ground literally at our feet.
The handhelds, if they light, all seemed to burn fine although there was a lot more splatter, dropping hot slag everywhere. Light and hold them way over the side if you use them aboard your boat. A couple had to be lit with a butane lighter as the striker material was too soft.
Just don't count on them if the chips are down.