1987 Chevy Monte Carlo water in the oil

Engine Performance problem
1987 Chevy Monte Carlo V8

my sister bought a used 1987 four winns boat with a chevy v8 inboard engine in it. From the time she put it in the water the performance got worse each time it was started and run which was only about 5 times. The last time it was run an oil/ water mixture was coming from the vents on the top of the valve covers. When I checked the dipstick the oil level was way past the full mark and as the engine cooled the oil on the stick turned to the oil/water mix color.
My question is- what has caused the water to get into the oil, head gasket no good or possibly something else?