Whats the best way to determin the source of water in the oil? Is there some way to pressure check a non closed system ?

The first thing to do is drain it all out and unhook the water pump, and run it for 30 sec with new oil ,,,do that for at least 3 times to get all the moist and water out.
U can just train the oil filter and reuse it for that part.

Then i would take it apart and see where u see rust ,that tells u where the water came from.

Don't run it longer then 30 seconds without the pump tough. Just so u get oil press. + a couple of sec. then shut it of and drain again.

The common thing is Exhaust or Intake !!!!
so before u take the engine out, take the exhaust off and check it for leakage.
Can be done easily at home on the water hose.

#1 manifolds or risers. remove from engine and pressure test. #2 Intake and heads remove from engine and check gaskets for failure. #3 block, when intake is off check for cracks in lifter galley

You did not say what kind of power you had in the boat. I just had to replace my stock risers on a pair of HP 500s for a pair with about a 9inch rise because i was getting water ingestion on th strbd motor. Its a cam thing on the HPs. Went through about 400 dollors worth of oil on that motor before the guys at AtoZ marine told me what the problem was. No more problems.

Had to do this myself recently. If you need to pressure test your water system, best place to start is by blocking off the exhaust hoses. Pull the hoses off the exhaust risers and either loop them back on themselves or just put a 'plug' in the hose. Then use your house water in the raw water intake hose to pressurize the system. Watch your water pressure gauge, I use a ball valve to slowly pressurize the engine up to about 20-25 psi and the listen/watch for leaks, your residential supply should be around 35-40 psi, hence the reason for the valve. Pull the spark plugs out before you do this, so if its the head gaskets you'll see the water coming out of the cylinders. If your don't immediately find a leak, leave it sit for awhile and check back off and on.