I had the exact same thing with my Westerbeake last spring. The raw water pump seal was leaking into the oil. I drained the oil and refilled five times. Once I had the oil looking good, and had changed the filter twice, I tried to start it - surprise the oil pressure switch was gummed up and got a face full of oil from blown oil filter seal.
The problem I has was getting the oil out of the engine. To begin I tried using the oil extraction hose on the bottom of the oil pan but three different pumps would not even move the milkshake looking oil/water mix. In the end, a simple drill pump pulled the oil out at a very slow rate. Each oil change, it got faster.
Once you get the oil changed and the water pump replaced, start the engine and let it run. I let it run 5 minutes the first time and then changed the oil / filter.
The next time, I let it run for an hour with the oil cap off so any moisture could escape.