Harbor freight borescope
 

I bought one last weekend to look under the deck at stringers and at the oil pan on my I/O. It has already paid for itself.

Of course now they are on sale. Or starting April 2. There is some other Easter sale this weekend, too.

Now it is $70 for the one I bought. I paid $95. Personally, I would get the higher resolution one if I had to do it again and had 2x the money, but this is pretty good. A LED lamp that doesn't have a sharp spot to it is a nice thing to throw in the bilge when you use it for better lighting, but this is handy.