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Actually, I'm not too worried about that bent backing plate. It's bent about as far as it can go, and there's virtually no tongue load, and what tongue load there is stresses the the rear U-bolt much more than the front one. Still, I'd replace it with something a bit heavier. You might have to spend $5 at the hardware store to get a piece from which you can obtain the 15 cents worth of steel that you need, but I'd do it, and maybe even add one or two more clamping attachments too (personally, I weld everything, but for people who can't, "more is better" is good advice when it comes to improvised connections like these). Sometimes, "trailer" and "loosening bolts" are practically synonymous.
The thing I noticed that's a bit worrisome is those two 2x6s standing on edge. They seem to have a bolt right through them edge-wise, but the flat edge of the frame they are bolted to is very thin metal and will have some "give" if the wood pieces decide they want to tilt one way or the other, and in any case, you are asking a very small contact surface to resist such tilting so even the softness of the wood will allow easy tilt of those boards. The crossbars across the top will only add a similar degree of support (just a little) against tilt. However, attaching another 2x6 "face-wise" across the ends of the existing boards, both at the front and at the back, would turn the whole boat framework into an extremely rigid unit with no ability for the individual components to flex relative to each other or to the trailer. It will even make the trailer more rigid. Stand on one corner of the trailer right now while it is hitched to your car and notice how much the frame twists, corner to corner. Do it again after attaching these extra boards, and you will see that the whole trailer will be considerably stiffer in torsion. All this extra rigidity would be excellent insurance against various connections tending to work themselves loose as the trailer bounces on a long trip.
Those are just some easy improvements to what's basically a really nice job. Not many people mess with making their own stuff anymore.