My Catalina 30 was doing the same thing because of two things: Engine started hard and anti-siphon valve was corroded. Put clean oil in the engine, close the water valve and start the engine. After the engine starts, open the valve. If you don't get water in the oil by doing this, then you have a problem with water filling the muffler. I found that in the Catalina 30 installation, if you crank for more than 30 seconds without the engine starting, the muffler will fill. My choke was not adjusted correctly-after adjustment the engine starts immediately and no more water in the oil