96 Sea-ray 190 w 4.3 merc engine problems?

I took my boat out for the first the first time this summer and although the engine was running fine, I noticed (after about 3 minutes) a light brown sludge looking substance foaming up all over the engine. The foam went away after a few minutes and left all this brown sludge everywhere. I put it on the trailer immediate but I'm not sure what it is. I live in TN and I winterize my boat, plus keep it in the garage. Could it be gear oil somehow from the lower unit or perhaps I didn't get a freeze plug back in correctly?