Not a stupid question so don't worry
I personally wouldn't top up with just water as you can't be 100% sure whether someone has been doing so for the last 10k miles for example. The antifreeze also stops the engine block (cast iron) from corrosion, which is (bar the fact that in extreme climates water freezes) probably one of the worst things about topping up with just water.
You'd be best to buy some antifreeze and mix this with water when topping up, as far as the mixture we're coming up to the summer so no need to go over the top, go for 30/70 meaning 30part antifreeze and 70part water.
I think you can buy readymixed coolant but it would probably be more expensive and is honestly really easy as it will explain how to mix it on the bottle.
Hope that helps a little bit,