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I have a 2005 Montauk 170. Put an hour meter on 2 years ago as this was a used boat with no hour meter. I change oil yesterday and drained about 1/2 cup of clean water (guessing) out of bottom plug before oil started coming out. Had just used boat for 1st time this year previous week. Last time used prior was probably October 2015. This is not 1st time of finding water in oil. Last changed oil 6/15/15 at 72 hours and yesterday with 97 hours on meter.
Is some water normal from moist air being compressed from engine running over water or should I be concerned of another cause?

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It's not normal at all, and continuing to run it with this problem will surely destroy the motor. Get it checked out ASAP. In the meantime, if your handy, pull the spark plugs and squirt some oil into the cylinders.

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Thanks

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Get it serviced ASAP. Normally a head gasket issue will shows signs of white foaming on plug tips or dip stick.

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Reading on web site, there is talk about condensation getting into oil from thermostat staying open and oil running cool and not heating up enough to stop water condensation. Is this possible and would it produce the amount of water I am seeing (1/2 cup plus/minus. I have talked to a repair man and he is checking to see if this was a problem in other mercury motors. He seems to think this will be a major task as you will have to check seal and gaskets to see if he can fin problem.
I will have it checked out, but I will also check thermostat and pull plug and see if I can see any rusting on cylinders.
What are you thoughts on the thermostat without having to tear motor apart?
Also, what is the price range of a new 90HP to replace if it is cost effective?

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I change the thermostat every 3 years in salt water, a bit longer for fresh so go ahead and change it out.

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With very minimal use you might expect some condensation, and some more if it either had a thermostat stuck open or was only used for very short trips or it never got a chance to warm up to boil off the water. However, half a cup seems like a lot.

At this point, I would change the thermostat, and maybe have your mechanic do a compression or other test to look for gasket leaks. If it doesn't turn up anything, go run it and keep an eye on the oil level.

If any internal damage was done by the water, it's too late to reverse it now - it either blows up, or it doesn't.

Whether it's worth repowering depends on the condition of the boat and how much you like it. Many Whaler owners would prefer to have a choice of outboard, so if you decide to sell it without a motor (presuming it blows up) I expect you'd have a ready market for it.

Tim

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Thinking head gasket. If it were mine I'd pull the head(s). Even if its not a head gasket (not sure how else you'd get a half cup water in the oil) you'll have a birds eye view of your cylinder walls which I'd want to inspect anyway.

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What did the oil look like? Was it milky? Unusual odor?

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After Pumping oil out 1st, I tilted motor and drained. The water came out clear, then the oil came out light brown then black with some light brown mixed in. No smell. The boat has bee sitting a week with motor straight up.
I plan on pulling the thermostat this weekend and possibly run a camera into spark plug port to see if any visible damage.
The mechanic is currently looking in web to see if he sees any info on this being a problem with this motor. If he can get to it soon, I will take to him and perhaps have him pull head to check out if I do not see any problems with thermostat or cylinder damage with camera.
I know we all hate to take a boat in for service as all we can see is the $$, but in this case I prefer not to have to purchase a new engine.
Thanks for all your comments and will try to keep you informed on outcome.
Thanks for the great site to get information from those who have experienced the same thing, keep it up.

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Update. I had a mechanic search problem. He pulled tge power head, then the cylinder head. He then removed oil pan, finding control arm parts and spline shaft had seized du e to motor being in a salt water environment before I purchased (runs only in freshwater now). Had to replace some of these parts as he had to cut them off to remove other parts.
He did find corrosion on the water pickup tube where oil pan flange connects to cylinder head. (terms may not be correct for cylinder head). He thinks that this corrosion could have cause the grommet to leak water into oil pan.
I am hoping to get to Lake this coming weekend to run boat. He suggested I change oil after I run it a little while in case there are fragment or trash from removing oil pan and also check to see if any water has gotten into oil.
He did not find any of the gaskets leaking.
I hope this post might help someone else in the future.
Thanks for all who responded.