As Martin has said, they are governed by exactly the same rules as any other trailer being towed by a car.
Actually with a boat trailer that gets dunked in the brynie (sea) I'd be more careful.
I used to have a 16 foot jet drive boat which I towed down the M5 to Minehead regularly. I used to keep a close eye on the wheels as they were only Mini wheels and running at top weight. One day I noticed a "tyre wobble" which I thought was a tyre about to burst, so I pulled over to the hard shoulder. You can imagine my suprise when on jacking up the trailer, the wheel and hub just plopped off onto the floor. Yep, the wheel bearings had given up.
Boat trailers usually have smaller wheels than most trailers and so the wheel is batting round a lot faster consequently bearings wear out quicker and also sea water corrodes quicker as well.
Also another tip on boat trailers is that the brakes often don't work as a result of salt water corrosion and hardly ever when the trailer is empty due to it's light weight unladen.
Where's the cops and their camera's just when you need them!!
Steve L.