Bolting floats onto an airframe changes its weight, balance, and performance in ways every seaplane pilot must respect.
Floats are heavy. A set of metal floats can add several hundred pounds of empty weight (often 150–300 lb depending on the aircraft), directly reducing useful load. Because the airframe also gains drag, the same engine now has to do more work.
Floats sit below and forward of the wheels they replace, which typically moves the empty CG forward and changes the moment arms. Float installation is done under a Supplemental Type Certificate (STC) that defines a new, often more restrictive, weight-and-balance envelope and a revised gross weight. Always use the float-specific W&B data and POH supplement, not the landplane numbers.
Seaplane takeoff performance is acutely sensitive to density altitude. High temperature, high elevation lakes, and high humidity all degrade both engine power and aerodynamic and hydrodynamic performance. A glassy, hot afternoon on a small mountain lake at 6,000 ft can leave you unable to get on the step or to out-climb the trees at the far shore. Plan with the float W&B data, compute density altitude, and respect the available water run length plus the obstacle clearance beyond it.
A forward CG makes the seaplane more prone to nosing in/diving and harder to get onto the step; an aft CG makes it more prone to porpoising and can delay or prevent planing. Load within the envelope, and bias toward the manufacturer's recommended takeoff CG.