Only 380lbs for the alumium trailer !
Not sure about other boat trailers...but...
My aluminum trailer floats! Just enough bouyancy in the tires and bunks to keep it off the bottom. This is good and bad. Bad in that it will go sideways (empty) when backing into the water. Good in that I can easily align it with my boat when landing.
(Yeah, I am one of the get-in-the-water-to-load-my-boat guys. I won't power the boat on to the trailer.)
-Eric