Antifreeze is not just for winter, though it lowers the freezing point of water, it also raises the boiling point too. Antifreeze/coolant provides protection from corrosion and rust and fights mineral deposits from water. Another side benefit to anti-freeze is leaks, it doesnt evaporate and dry quickly plus it is dyed so unlike water it will leave more evidence behind when leaking.
A stronger water mix ratio will provide better heat transfer but I would at least try to run around 25-30% anti-freeze just for the other benefits. There are also additives you can put in the system in lieu of anti-freeze to supplement the water or supplement the heat transfer properties.