How to fix a water leak?

Thick, white smoke coming go out of the exhaust is an indication you are burning coolant. You just confirmed that is coolant by checking the oil, as you have coolant mixing with the oil. That happens when the head gasket on the engine fails. The fix requires taking the engine apart to replace the gasket. It is labor intensive which makes the repair expensive. So a mechanic will suggest installing a used engine to save money. Your looking at a ballpark repair bill of $2K. I would not continue to drive it, except directly to the shop. Overheating the engine is not good. Good Luck with the car if you intend to repair it.