As part of a larger emerg kit, I just made an SOS kit for a boat, designed to be small, inconspicuous, float and ALWAYS be on board, even if the larger kit is removed. I put key basic emerg gear (fire, water purification, signalling, sheter, navigation, first aid, light and food) in a Nalgalene. I put that in an aluminium can and duct taped them together. A metal vessel for boiling water was one of the key items I wanted to include. The can was cheap and easy.