Remember your #1 priority. Clean Water!
These allow you to filter out bacteria and protazoa from any stream or pond or lake. They do NOT turn salt water into fresh water. If you also want to filter out chemicals like chlorine, mercury or lead then the bottom ones with a charcoal filter are what you need. When an emergency arises, it makes no difference if you have food and solar power. If you don’t have available water that won’t make you sick… You won’t survive long.