Very good point. Add this as #5 on the list.
#5. Back-flooding through the exhaust system due to failure to rapidly start the engine.
Yep... overcranking due to things like a partial hydraulic lock (or for a number of other reasons) can push water down into your crankcase. It comes back through the mixing elbow, into one or more cylinders and then gets compressed down into your crankcase. If it takes more than 4-5 seconds to start, this could be your issue. A couple of things to check for (if, and only if, hard starting occurs). First, after you crank the engine for 5-7 seconds and it doesn't start, STOP trying to crank it. Loosen up the dry exhaust output from your cylinders. If water comes out you've probably found the source of your water. Also, do a compression check between all of your cylinders. If the cylinder closest to the exhaust has lower compression than the rest then most likely you've been having back-flooding issues. Please keep in mind that water in the crankcase is a SYMPTOM of another problem - not the problem itself. You stop the water by tracking down the back flooding issue, which may be caused by things like a clogged vented loop, or a vented loop below the waterline, in your exhaust system.
I would highly recommend getting your oil tested. You can call Mack Boring up and ask them for a test kit. They will ship you a kit, you ship it back to them, and they will fax/mail you a breakdown of everything they find in the oil (what kind of water, coolant, metal, etc.). It's very handy for tracing down problems and they give you a decent description of what causes the levels of contaminants in the testing.
The other thing you can do to verify if it's actually water is the crackle test. Put a few drops of oil on a piece of tin foil and then hold a lighter under it. If it crackles/pops after a bit, there's water in it.
P.S. I agree with putting this as #5 UNLESS you have a hard starting issue, in which case I'd put this as #1 on the list.