This can happen to ANY trailer, not just boats. It also can happen to semi trucks.
Threaded fasteners are designed and manufactured to be used ONCE.
In many application cap screws (bolts) are used again on motors and various frame work. The problem is, that bolts when torqued to the specification of the size and rating of the bolt and the job its intended for, reaches its maximum clamping force. Do that again, will stretch the bolt which distorts the pitch of the threads and causes the clamping force to be reduced, which allows most Hex nuts to back off. Not only that, people who do repairs at home, often end up with the wrong hardware. Capscrews and Hex nuts are rated based on the ID symbols stamped into the part by the MFG. A lot of fasteners are imported and there have been a lot of counterfeit fasteners on the market for years. If you use a typical grade 2 nut or stainless steel nut on a grade 8 or grade 12 bolt, you will not achieve the specified torque rating of the bolt and the nut will be deformed or stripped. Nylocks are common in the hardware stores, but if stripped the plastic collar inside the nut is not enough to keep the nut on. Stainless has the strength of grade 2. Most auto hardware is grade five, but since most fasteners are now metric, the typical ratings are 8.8 = grade 5 and 10.9 = grade 8. Metrics stamp the numbers on the head but the hex nut is literally stamped with what resembles a center punch and a hash mark at least on the american made grade 5 and 8 hex nuts. If you buy a cheap flat washer for a high torque application, the washer will flatten and reduce the torque rating.
When in doubt go to tacoma screw or high strength bolt or another commercial fastener retailer. When it comes to UBOLTS you should buy from the manufacture or go to a spring retailer. They sell and make coil and leaf springs. A retailer like 6 robblees is reliable and NAPA but if the counter guy cannot tell you what you are buying, then go somewhere else.
http://www.offroaders.com/tech/Bolt-Identification.htmIn a personal experience, I rode with a boat owner and fished the salt. At the launch I noticed his axle had moved from the center position on the leaf spring, on the drivers side. It was old and rusted. He failed to do proper maintenance. We tightened the U bolts at the launch but on the highway, the axle shifted again. That allowed the tire to rub on the fender, which heated the tire. The tire exploded and ALL FIVE lug nuts were stripped off the studs and the tire and rim was gone. We dragged the hub of the axle on the freeway until he stopped the vehicle. Fortunately he had a spare and we robbed a few lug nuts off the other axle so we could get it off the freeway and then dropped the trailer and bought some more lug nuts and U bolts.