Don't sweat it, change the oil, replace the manifolds and get it running ASAP. The important thing is you need to run the engine very soon and get rid of the moisture. At the bare minimum (as of right now) I would change the oil, remove the water supply hoses going to the manifolds, hook up the muffs, and start the engine letting the coolant water run into the bilge until the manifolds start to get hot, then shut it down before you melt the big exhaust hoses. Do this for now until you get your new manifolds installed. Then once your new manifolds are on, take the boat out and run it easy for the first hour or so. Take your oil cap off and look for moisture, milky oil, etc. If none is present, then your good to go.
