There are a variety of fire/water proof document sized bags you can get on Amazon in the realm of $10-20.
Personally I have a fire/waterproof safe with a number of important documents and a fire/waterproof bag within that safe for the *super* important things.
It might seem silly, but there is something to be said about the peace of mind something like that can offer - the last thing you want to worry about during something like a fire or flood is saving documents over family/pets.
My recommendation long-term is a safe (depending on how much/many important things you want kept safe) or at least a water/fire proof bag of sorts.
Personally, I got this bag which sits inside my safe though I (thankfully) cannot speak to its effectiveness.
So just a waterproof bag? This is just the first one I found in 10 seconds of searching, there are like 20 other versions of this on Amazon for under $20