I discovered this weekend (after completly overhauling everything else on the starbaord engine over the last 2 years) that if the raw water pressure regulators are stuck shut you will get steam from the exhaust at engine speeds below 3000.
The quirky thing was that as long as I stayed below 3000 RPM the engine never offered to overheat. It never offered to overheat at slow speeds either.
This year after overhauling the running gear, and finding that the engines would spool up to 3800 I discovered that 3200 or faster with stuck shut regulators will cause overheating with a vengance.
After getting them out and soaking them in WD40 I still had to tap them with a gasket hammer to get them free.
This does not explain the clean plug, but it is an additional thing to think about. It is also something I would have never even thought about without this site, Thanks Paul.