If the oil is milky then water is getting into the engine. There are a couple of ways for this to happen.
Least likely is a bad lower seal. If you don't see any oil on the lower exterior of the engine or coming out the lower unit I wouldn't look there.
Next would be a crack in the block under the cooling water jacket. This is a not uncommon thing in some older 130 hp engines, but not the 90.
What I do see is a leak in the head gasket. Don't just pull the head gasket though. You need to do or have done a leak down test. A leak down test puts pressurized air into each cylinder one at a time through the spark plug hole. A leak down tester is then used to measure the amount of air escaping. This should show you which cylinder is leaking.
Only after finding a cylinder that fails would I pull the cylinder head.
You can buy a tester from most automotive tool suppliers. You will need an air compressor as well.
If the cylinder head passes the leak down test then I would suspect a leak elsewhere, particularly under the water jacket.
One other thing to keep in mind is depending on your water and air conditions and how you are using the boat, you can get quite a bit of condensation in the crankcase. This condensation can then accumulate, particularly if the engine is not run long enough or if it is running too cool or too much idling.
If you suspect this, do a oil and filter change run the engine at normal speeds, as long as it isn't running bad, for at least 30 minutes and then recheck the oil. Also see if you can distinguish between oil with water or oil with gas in it. If it is gas, then that's another problem.
Let me know what you find and post back and we can go from there.