Help with strange type of bolt
My guess would be a carriage bolt, but it's impossible to tell for sure.  Take the nut off the inside, hit the bolt with a mallet and see what happens.

Edit based on addition of interior photo...

I wonder if that big black cylinder is some type of damper intended absorb shock and vibration.  Maybe the one on the other side failed and they replaced it with a regular bolt.  You might try a channellock pliers or such to take it off (if that's your goal).  Or post this to a boat forum and see if anybody has a better answer.