Tho ideally you should have a thermostat valve but it has nothing to do with water level. It's either a leak in the system or could be a blown head gasket. Let's hope it's not the gasket.
Here's what I think you should do:
1. Change all the pipes/hoses.
2. Get your water pump checked.
3. Get your cooling system pressure tested. (Only if you're still losing water.)
4. Install a (88 C, I think that's the correct value) thermostat valve.
Don't listen to local mechanics, they say weird things all the time.
Additionally, I'd suggest using Caltex coolant with 50% distilled water but any good quality coolant will do the job.