it could be a dodgy senderl change the sensor that fits into the exhaust/ turbo side of the head - they are about a £10

1/2 spanner, remove and have the new sensor in your hand ready to plug back in - as water will piss out. Do it when the engine is cold

also i wouldnt run a cossie just on water; get at least 50/50 split - water will just boil and evapourate!! - get that changed

personally i'd always put a new stat on any new engine to be on the safe side - i assume the stat opens?

i assume the pump was OK?