One presumes this question refers to an auto engine, and that you are describing water found in the oil. If so, the problem is not a small one. Either the engine block is cracked (most serious) or a head gasket is blown (still serious). Another possibility is a warped engine block and/or head.
A head gasket can be replaced by a good mechanic but depending on the vehicle, it can be quite expensive -- but lots cheaper than another engine or vehicle. If the block is cracked, again, this is quite serious, in many cases requiring a new or remanufactured block.
Auto parts stores sell a variety of products intended to repair head gasket leaks -- including engine sealants -- which may or may not work. Even if they do work, the "fix" probably won't last very long -- but it's probably worth a try.
A different problem is oil found in the car's coolant. Some cars have oil coolers mounted inside radiators, so if the cooler leaks, oil will float to the top of the radiator and form a visible scum. The fix is to replace the radiator.