I also thought about this years ago. mostly for a masthead float for a
trimaran. I considered using calcium carbide that reacts with water to
produce LOTS of acetylene gas that would seriously inflate some kind of
bag. Of course acetylene can be explosive with a spark but you are
sinking anyway....... The calcium carbide would be fairly easy to
store as it comes as rock-like chunks but produces copious volumes of
gas.

I have also considerd adding foam floatation to my boat. She has many
little areas that are never used for anything whose total volume might
be enough.