Can you get to the underside? If so you need to through-bolt all the stanchions with bolts and backing plates or at least big washers. There's a LOT of strain on them. You could use lag screws the next size larger (3/8"), but the problem will likely recur.
You may be able to find a "blind anchor" that will work. The old "Molly Bolt" comes to mind if you can find a suitable one. There are other types.
If they were loose there's a good chance you've had water intrusion and possibly rot. This needs to be repaired first. Then fill the holes with epoxy and re-drill.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw
07:32:12 AM by