I don't think it would hurt the axles so much at about 40lbs per axle BUT I would be concerned with being able to properly secure it down.
What I would do if the only place I could put the boat would to be underneath the trailer would be to take the trailer to a welding shop and see if they could weld 3-4 braces from each side of the frame to create a under TT storage rack to put the boat in. Once you get it framed in then you could attach some sheet metal yourself to enclose it and create a compartment with some tie down spots.
