We looked at a 2008 model of a Hunter over the weekend, always in fresh water. It was nicely equipped by the previous, now retired and moved away, owner to single hand. When we opened up the boat to look down below it smelled "funny". Not a mold smell, not mildew, not holding tank and not diesel fuel. I pulled up a couple floor boards to find a small puddle of orange/pink yick in the shallow bilge, it had come from the direction of the diesel engine and hot water tank. In the middle of the puddle were two very rusty and corroded keel bolts. This really surprised me because none of our boats ever had an issue with rust or corrosion on the keel bolts. I figured that pink could have come from the RV antifreeze used to winterize the fresh water system, I know that is not corrosive. The orange could have come from the coolant used in the diesel, once again if someone pulled the wrong hose off the hot water tank it would have leaked coolant. I don't think either of those chemicals corrodes metal, in fact I think just the opposite. I "assume" the keel bolts would be stainless so even water in the bilge would not rust them? The only other thing I could think of was battery acid. I know this boat has two group 31's located aft of the diesel, although I'm not exactly sure where. We were "visiting" the boat kind of unsupervised so I didn't think tearing out the aft cushions and locker boards would be appropriate yet. Seems that a battery acid would smell and corrode? I was thinking that maybe the batteries were left on board all winter, lost charge and froze. That theory has yet to be proven or disproven. If we decide to go this direction I will have the boat surveyed of course, but I want to understand a little more about what we could be looking for.