There are several entries in the aircraft.cfg required and optional:
Contact points for floats are class 4 and rudders are class 5 as below (see my D18S floats as example)
point.1=4, 11.25, -6.417, -6.90, 3000, 0, 0.00, 0.00, 0.85, 2.50, 0.65, 0.00, 0.00, 0, 0.00, 0.00
point.2=4, 11.25, 6.417, -6.90, 3000, 0, 0.00, 0.00, 0.85, 2.50, 0.65, 0.00, 0.00, 1, 0.00, 0.00
point.3=4, -2.50, -6.417, -6.40, 3000, 0, 0.00, 0.00, 0.95, 2.50, 0.65, 0.00, 0.00, 2, 0.00, 0.00
point.4=4, -2.50, 6.417, -6.40, 3000, 0, 0.00, 0.00, 0.95, 2.50, 0.65, 0.00, 0.00, 3, 0.00, 0.00
point.5=5, -12.5, -6.417, -6.50, 1600, 0, 0.00, 25.0, 0.00, 0.00, 0.65, 1.00, 1.00, 0.00, 0.00, 0.00
point.6=5, -12.5, 6.417, -6.50, 1600, 0, 0.00, 25.0, 0.00, 0.00, 0.65, 1.00, 1.00, 0.00, 0.00, 0.00
Also, in the Gear_Warning_System, the SDK says:
gear_warning_available = 1 Sets the type of gear warning system available on the aircraft. 0 = None, 1 = Normal, 2 = Amphibian (audible alert for water vs. land setting).
In the Effects section, the SDK has this:
Wake: The wake effect triggered behind float planes
Water: Water splashes triggered at low speeds on float plane
waterspeed: Water splashes triggered at high speeds on float plane
In the Sound.cfg, the SDK points at terrain type flag=8 to control sound type.
Of course, the model must have the floats and rudders modeled in.