I'm not sure how the first mechanic described it, but "water in the engine" is a crock. As mentioned by the second, the Headgasket may have blown. When it fails it allows coolant and oil to mix. Check your oil and look for a milky brown consistency. Theirs a reason why it overheated in the first place, and a blown headgasket is usually a result of overheating, not the cause. Some causes of overheating include the fan clutch, the thermostat, and the water pump failing.