In case anyone is interested, I came across a mod for the default Beaver that allows you to use it as an amphibian. It modifies the contact points and makes no graphical changes. You'll need to assign a key for gear or use the default G key. If you look at an outside view, it looks like the plane is sitting off the ground on it's floats. But if you just want to use the default Beaver as an amphibian from the cockpit, this will do it!
Replace the contact points in your aircraft.cfg with this. (back up original)...
/// Default Beaver-Mod to allow amphibian ops. (non graphical)
[contact_points]
max_number_of_points = 21
point.0 = 1, 0.8, -4.80, -9.33, 1600, 0, 1.40, 45.0, 0.15, 2.5, 0.80, 4.0, 4.0, 0, 175.0, 175.0
point.1 = 1, 0.8, 4.80, -9.33, 1600, 0, 1.40, 45.0, 0.15, 2.5, 0.80, 4.0, 4.0, 1, 175.0, 175.0
point.2 = 1, -10.55, -4.80, -9.485, 1600, 1, 1.50, 0.0, 0.4, 2.5, 0.80, 4.0, 4.0, 2, 175.0, 175.0
point.3 = 1, -10.55, 4.80, -9.485, 1600, 2, 1.50, 0.0, 0.4, 2.5, 0.80, 4.0, 4.0, 3, 175.0, 175.0
point.4 = 4, 0.00, 4.80, -8.00, 2200, 0, 0, 0.0, 0.0, 2.5, 0.390, 0.0, 0.0, 3, 0, 0
point.5 = 4, 0.00, -4.80, -8.00, 2200, 0, 0, 0.0, 0.5, 2.5, 0.891, 0.0, 0.0, 2, 0, 0
point.6 = 4, -18.10, 4.80, -7.00, 2200, 0, 0, 0.0, 0.5, 2.5, 0.891, 0.0, 0.0, 3, 0, 0
point.7 = 4, -18.10, -4.80, -7.00, 2200, 0, 0, 0.0, 0.5, 2.5, 0.891, 0.0, 0.0, 2, 0, 0
point.8 = 5, -20.10, -4.80, -7.30, 1600, 0, 0, 50.0, 0.0, 0.0, 1.000, 1.0, 1.2, 0, 0, 0
point.9 = 5, -20.10, 4.80, -7.30, 1600, 0, 0, 50.0, 0.0, 0.0, 1.000, 1.1, 1.5, 0, 0, 0
point.10 = 2, -6.90, -25.0, 4.50, 1800, 0, 0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5
point.11 = 2, -6.90 25.0, 4.50, 1800, 0, 0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 6
point.12 = 2, -30.23, 0.0, -0.00, 1800, 0, 0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 9
point.13 = 2, 0.48, 0.0, -2.17, 1800, 0, 0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 4
point.14 = 2, -27.50, 0.0, 7.50, 1800, 0, 0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 7