If your engine was running when you used the garden hose, you should not have gotten any water in the engine. I believe your m50 has an aluminum head. If it has ever been overheated, the head bolts should be re-torqued. I would do a coolant system leak down check. You usually can borrow or rent a tester from Auto Zone or Advanced Auto. If it leaks down with no exterior signs of leakage, it is probably time for a head gasket. If you replace the head gasket, replace the timing belt. The belt must be replaced at a certain number of engine hours or you run the risk of engine failure. Very $$$.