Happily, I have no personal experience of this issue. However, I have read various texts, safety bulletins, etc. suggesting that for most light aircraft, the question of grass vs. pavement is essentially academic: provided that the pilot keeps his cool and doesn't stall in or land extraordinarily 'hot', a 'gear-up' landing is almost always survivable, and usually results in modest or even minimal damage. I have several friends who have inadvertantly landed 'gear-up' on grass, and their experience bears this out.
The issues associated with landing a floatplane are totally different to those associated with one which is not ordinarily designed to land without its wheels
I don't have extensive experience flying over water in a landplane, or flying over land in a seaplane. However, I do know that I have always felt much, much more comfortable in the latter situation. Properly handled, a floatplane can make an emergency landing almost anywhere (including many places a landplane couldn't handle), often with little or no damage to the floats. Again, I speak without personal experience (touch wood!), but this has been demonstrated many times and is supported by many authorities (Faure, Frey, Kurt, etc.).