How did the sealing surfaces of the exhaust manifold and riser look? If they get eroded you can get water in a cyl via the exhaust manifold but you said you did not see anything but black carbon. Well a good test for water in the cyls is....warm it up...disable the ignition....remove the spark plugs and crank it over. If you had water getting into a cyl from the exhaust or a small head gasket leak, it will be obvious because it will blow out the spark plug holes. Keep in mind sometimes its just a spray or mist of water not a lot. When my engine blew both HGs, the #2 cyl had water that would shoot out of the cyl, the #1 just a mist of water. Both plugs had a slight orange-ish cast to the porcelain center insulator.
You can check your exhaust manifold by taking it off the engine, draining all the water out and propping it up level. Then replace the drain plug and fill it (CAREFULLY, flammable) with Acetone. Let it sit. Look at the ports that line up with the exhaust ports in the cyl head. If you see any dampness, then that's a leak, either a crack or rotted through area. I did this with my OMC batwings several times. When I had water in the cyls, I checked them this way first, and they were fine. I then rigged up a way to check for combustion gas getting into the cooling water, after seeing it mentioned in a Volvo Penta overheat diagnosis document. I removed the thermostat and rigged up 2 funnels in the exhaust manifold feed hoses. You can't run it long this way because the exhaust isn't getting cooling water. However, at the end of the vid when I shut off the engine, you see bubbles rise up in both funnels. That was combustion gas getting in the cooling water, and water leaked into the cyls as the engine cooled off. Watch it all the way to the end.....
https://www.dropbox.com/s/8lz4p4e7pd...20232.MOV?dl=0
When I took the engine apart, the head gaskets were clearly blown, at the fire rings. The block deck looked fine but the cyl heads after being checked by a machine shop, had cracks in the center cyl exhaust seat areas. And the machinist felt that the cooling ports were eroded by salt water use and would not seal if the heads were decked after being repaired. So I installed a set of reman V6 marine heads.
