I'm sure that using one of those flares would puncture my SIB
We fire flares all the time (training), and although it might sound obvious, this is what we do, you would be amazed how many people fire flares from inside the rib/boat.
We find the flare we want to use, usually a white para flare for searching, remove the end caps, and then Kneel on the floor, or up against the sponson and sort of lean out over the edge of the boat, safely of course. With the bottom of the flare facing the sea, you can then set it off.
IF it goes tits up and the flare backfires, its only going to go into the water, also dont forget there is sometimes a bit of a backdraft or 'bits' flying around when you set it off. You dont want these going off near your sponson
So everything happens over the water.