If you divide cubic centimetres by 1000 you get liters or kg, if you want metric ton divide by 1000000 or just use meter not cm.
DE is used in vehicles where loads and speed varry most of the time. Like tugs and freight trains. This is not the case with most recreational...
http://m.youtube.com/#/watch?v=lYPFATFM9ZA&desktop_uri=%2Fwatch%3Fv%3DlYPFATFM9ZA&gl=NL...
In your drawing the gantry is orientated 90° from your pontoons, in this way your couplings wil have to bear all of the forses.