Re: merc exhaust plate failure
As TD said, the baffle, inner water jacket cover and outer water jacket covers can get holes, warp or the gaskets simply fail. They will allow water to get into the cylinders. If you have water in the bottom one or two cylinders, it could also be the lower crank seals.
The repair is not hard, unless the bolts are corroded into the block, like they usually are. Then they break and the fight starts....