Water leaking into the engine's oil is caused by a blown head gasket or a faulty intake gasket. Most engines require coolant to be used in their cooling systems. When leaks occur in the system the oil consistency changes into a milky brown color.
When there is coolant contamination present in the oil it usually means the head gasket is blown. During the presence of a coolant leak, the cooling system drops in pressure, which causes the engine to overheat.
Another cause of a leak could be due to a damaged intake manifold gasket. If the intake has ports for coolant lines, the gasket can pop, causing contamination. To repair this problem the gasket will need to be replaced, the oil drained and the coolant flushed.