That's pretty poor construction. Unfortunately, it's not the first time I've seen that on here.
The best fix is going to be some kind of backer plate or even a couple of large washers + lock nuts + 3M 4200.
That will be dependent on whether you have access to the back of the swim deck or not. On my 87 Saltare, this involved me cutting out the floor beneath the fuel tank. It wasn't a lot of work and it's not visible..... but I wouldn't want to do it in my storage/sack locker on a V-Drive. It's going to be messy.
The next best fix is going to be some kind of anchor. I don't have any experience with these fixes but either a blind rivet nut or a well nut should work. I think it's at least worth a try. The well nuts are inserted from the outside and the rubber bushing expands as they are tightened. You'll need to measure the hole in the bracket and by the correct size. http://www.mcmaster.com/#well-nuts/=u8xvru
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.