Anything over 20kts in a 206 unless the water is really sheltered will be pretty rough and as for leaving it in the water overnight in rough strong winds - unless the floats are sealed completely (they never are) I wouldn't do it. Most of the water when sitting on a mooring in rough water will come over the top of the floats and come in through the access hatches.
Yes they can taxi up onto hard packed sand as long as you are shown how otherwise you will spend some time digging the plane out and or waiting for the tide to come in!
For a heavy 206 you will need about 0.8 M of water to operate - 0.5 of water under the float keel is a minimum I would ever accept.
Amphibs are poor performers (particullay the 206) so don't expect to fill all 6 seats and go flying.
Make sure you get yourself a good endorsement not a quickly or cheap one as it will only bite you if you want to operate one in the log run.
PM me if you want more info.