Speaking from experience, I have had two real autos onto the ocean, and I do training in a dam once a year. All have been in a MD500. The landings are very soft, oleos must help out here aswell.
The only thing that you have to think of is the wind, no power lines, trees or school yard filled with kids to worry about.
And yes, run on autos are not really the desired auto for water, and if you are being silly and doing running boat like take offs, watch out as the front of the floats will try and be a submarine.
Just do some normal landings on water, take it easy, have some fun and as they say ' you only learn from experience'.