1. Water landings..... somewhere around a couple of hundred.
Hours on floats to be more or less employable........ circa 50 odd (will all depend on the company, and just what they want of their pilots)
Open water is something that no employer will send you out to fly on unless absolutely neccessary. Believe me you can have enough "harder type" landings in even the most enclosed areas such as Sydney harbour (Rose Bay etc), due to the continual swell and wash created by other water traffic.
Not sure on the PPL/CPL ratings (if different), but would tend to believe no differences due to the logbook entry simply stating "float alighting gear", as we Aussies call it.
2. Fuselage of a floatplane is usually had much more anti-corrosion prep work done to it, and has quite often had extra things like skin doublers and what is known as a "float kit" fitted. This is a kit that includes extra braces, stiffeners and other bits and pieces such as stainless cables and some more corrosion proof fasteners fitted also. Some a/c even have STOL kits fitted to lower the float speed on the water's surface (hydrodynamic drag).
The W+B is re-calced to take into account the effects of the wieght and the aerodynamic effect of the floats on the stall, but
for the most part the MTOW is much the same as for the landplane version of the same a/c, however this is not always the case. Yes, floaties can usually be "converted" back if required to landplane status (though why you would is a question worth asking).
Permission to land is not
usually a problem, however some areas are much more noise sensitive than others. Use your commonsense here. Fortunately we haven't gotten to the stage that the US local authorities have in relation to regulating water usage and access.
As it is still a boat while it is on the water's surface, you will be required to have a normal boat drivers licence before you operate off the water.
For the American story on how bad the regulations can be see....
The Seaplane Pilot's Association
There are also many useful links and stories there.
Regards,
OpsN.