When I was 23, I was a dock-hand for a boat-rental club. I bought myself a 27ft Catalina sail boat and lived on it at the docks for about a year while I worked for the boat club. I would often get toasted on 101 proof peppermint schnapps and go joy-sailing late at night on the Chesapeake Bay for kicks.
One night my main sail tears, and my atomic 4 engine breaks down. I drift out of the channel, drop anchor, plug in my back-up batteries to power for my anchor light, and bed-down in my forward berth to wait until morning for one of my co-workers to tow me back in.
I'm about a mile offshore, well out of the channel in about 60ft of open water when I hear a rhythmic thudding on my hull beneath me. It was like someone was doing a semi-fast snare-roll with closed fists against my hull. There was nothing in my head that I could figure could make that noise happen besides someone diving under by boat and literally beating on it.
I went topside with my flash light to investigate and couldn't see anything out of the ordinary, and the sound continued on and off for about 15 minutes then stopped. It was a calm night with nearly no wind or waves at this point, and I visually couldn't see what could have been making the noise. It came from mid-ship so it couldn't be the motor being weird or something, and I checked my bilges for any anomalies and couldn't find any.
It left me pretty shaken up because I just couldn't figure what could make that sound as loudly, and as precisely as it was. I could FEEL the bumps hitting against the fiberglass hull. I eventually got back to sleep and made it back to the docks next morning. I dove on my boat that day to check for anything amiss and didn't find anything off. Needless to say that was the last time I went out alone at night..