Could be from a bad oil change?

Could be a little leaking from the motor somewhere. But you said you are not low on oil.

If you trailer the boat, get some Dawn Soap and a garden hose down there. You should be able to get under the motor with a sponge or small brush to clean real good.

Then just tilt it back and most all water should rain out. I've used a floor jack to help it tilt.

Plus that old dirty water can get smelly down there.

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