I'm currently having the same problem with my boat. Replaced waterpump, thermostat and impeller even thou all the parts were replaced last season. Also replaced radiator cap with the correct volvo cap. Pulled the heat exchanger and flushed it out. No problems there. No water in the oil, no...
Did you ever find out what the problem was? Im having the same issue with my volvo penta 125a. Steam's out of the antifreeze cap after running for a couple of minutes.