I will not argue a single bolt is stronger/better then a u-bolt of similar dia bolts.. It’s not stronger, as for better ????. But a single bolt eye is probably easier to install.
That said I think a single bolt of 1/2" to 5/8" Dia with a proper backing plate is more then adequate. Well..unless you plan to hang the boat from the rafters of your pole barn by the bow eye that is. If one shears the threads of the nut or bolt off, or shears the bolt shaft itself, then they are probably doing something very wrong.
As to the concerns of the nut coming loose… well they make this stuff called “thread locker” I usually use the Loctite brand “Blue” when I may have to remove it in the future…and “Red” if I never plan to remove it, but still want the option to be able to. I can not remember one instance since I started using Loctite back in the late 80’s – 90’s when I’ve had a nut “work” itself loose once thread locker was used. If you want to be extra safe though…just use some 2-part epoxy as thread locker…but then you’ll have to cut it to get it off. But it won’t work loose.
They key thing for mounting either type of bow eye is to ensure your backing plate is secure and large enough to transfer the load over a wide area. It was mentioned previously and rightly so. A properly installed backing plate is more important then which type of Bow Eye Bolt is used.