If you can't tighten the bolt, it's threaded, or nut is missing. Just try to remove all the bolts and inspect anyway. The boat looks well used so it would be a good idea to pull everything apart to inspect and/or replace anyway.

In the photo, I think you'll find a bolt head if you stick a screw driver down that hole in the beam.

Try to find the nuts. Or stick a camera in the hull to take a photo with flash on. Or try to take the beam end cap off and shine a touch into the beam to see the bolt head.