Guys:

I have a 94 disco and I am losing a small amount of water. I could not find any leak in the car. This weekend, I drove it about 400KM (like 300 miles), and it had lost about half gallon in the water deposit.

My mechanic says that it is not the head gasket, because the headgasket consumes the water very, very quickly.

What do u think?

 

I hope that you also have the water mixed with antifreeze.

You could remove the spark plugs and inspect the tips any that look very clean or that have rusted electrodes indicate water ingestion.
The coolant basically steam cleans the the plugs so it removes all the combustion byproducts.
You could also go to NAPA and get a block tester its 50.00, and it samples the vapors from the coolant bottle while the engine is running, this vapor passes through a blue liquid and if there are hydrocarbons present then the liquid turns yellow.
You could also get a pressure tester and see if there any visible leaks , do this with the plugs out and the engine warm then crank it over and see if any coolant comes out of any of the plug holes