- How would high winds in rough but sheltered water affect the aircraft while moored?
Wind will normally weathercock the plane into it and is not a huge problem. Leaving a float plane on a mooring unattended often is a problem though. The ones I fly are not insured when unattended in water. Hence we take them out of the water every night. In the case of an amphib it would be flown to an airport and landed wherever this was possible/economical or taken out like a conventional float plane if there was no airport nearby. Most floats leak in water so long term unattended risks a sinking. High winds would only speed up the ingress of water. More than one operator has found his/her pride and joy upside down with one float full after leaving it unattended.
- How much water depth would typically be needed for the plane to float?
Depends on the float and aircraft attached to it and the weight in the aircraft. Operations in water less than a metre deep would be risky in small float plane.
- Can amphibs reliably taxi onto hard pack sand embankments from water for the purposes of loading?
Dunno, never done it, I would think it would be a bad idea to try. I would load with the tail of the floats just out of the water on the edge of the sand with the undercarriage up. A correctly graded concrete ramp is all I would attempt to taxi up from water onto and only if it was an approved procedure for the particular setup.
- Are amphibian floats and straight floats separate design endorsements?
No but you need a retractable undercarriage endorsement for an amphib.
Download this book, it is free and a float plane bible:
http://www.faa.gov/library/manuals/a...-8083-23-1.pdf