u/Appropriate-Day8135

I drive a 98 grand cherokee limited with a v8 engine, ~20 gallon tank and recently it has been turning off while im driving it. It's fine when im going over 20 mph, but as soon as I slow down or stop it'll turn off. All that will be on when it does this are the radio and lights, steering and acceleration go out completely. Everyone is telling me I've got water in my gas tank, which is very likely due to the people I got it from leaving it for years with half a tank. Would this be something I could easily/cheaply fix? Is this what happens with water in the tank? I've put a red heet bottle in last time i got gas, but that hasn't done anything yet.

I've been told to get a fire extinguisher for my car, but can't find one that actually has good reviews and doesn't cost 1 billion bananas. I do have a fire blanket, but have been told a fire extinguisher is still necessary. Any recommendations? Preferably one that won't harm the environment or mechanical parts!