The most common cause of this that I have observed is a "stuck thermostat".
Thermostat is typically around where the water pump is. You can find more specific info in your owners manual or service manual, or buy or borrow a service manual at a local public library or auto parts shop.
Thermostat normally holds closed when the engine is cold to prevent water circulating, and then when it gets hot, the metal bends open. If something is broken it may stay stuck closed when the engine is warm, so then the result would be your engine overheats if you drive for longer than 15-20 minutes at a time, and your interim workaround might be to just drive short distances and add water every other gas station you pass.
It could also be a bad water pump. Try replacing the thermostat first, and if that doesn't solve it, also the water pump. And of course check the belt to the water pump and make sure it is in good condition, of course a broken or slipping water pump belt can also prevent proper cooling.
And some other possible causes are plugged or cracked radiator hoses, or leaky or plugged radiator. Replacing radiator hoses might just be a good idea if they look cracked or abraded.