Hopefully its not 'Water' that is in the system but a good Ratio of Antifreeze, (Coolant, summer coolant, rust inhibitor)
This will have it running at the correct temperature hopefully.
Get it Bleed properly with the correct coolant in, then if still a problem, its going to be down to tracking down the leak.
Radiator and hoses would be the thing to check first before the more involved stuff.
george