Re: water leak between lower and upper unit
Its just the water that is coming down from the engine, on its way down through the exhaust shaft and down through the prop, theres supposed to be a plastic (or metal if forget) insert with a big rubber seal that is supposed to keep the water from escaping through that small gap on its way down. They crack dry rot people forget to put it back in these things happen.
The only possible thing you could have to worry about, and this is an ultra slim chance, that the hot exhaust water could be coming in contact with the water pump housing, which sits right behind this insert and seal.
I wouldn't worry about it as long as you aren't having any cooling problems.