If the aircraft floats at a weight, the floats are supprting 100% of that weight, irrespective of what it is. The floats will have been designed to provide X kg/pounds max bouyancy each. Exceed that, and the aircraft will sink, irrespective of its BEW or MTOW.
If, however, you had said the floats are designed to support 180% of the aircraft's MTOW......