For inland water such as lakes, reservoirs and tarns there's a twist. As I understand it no-one can own water but like fishing, where Riparian Water Rights control usage, it's the banks that can thwart you.
In other words, if the landing and taking off can be said to be 'reasonable' access to the water should be permitted. The rub is finding a place to beach or moor.
David west at Loch Lomond Seaplanes has to be the UK's leading authority on the subject so maybe an e-mail to him might help.
I used to really want to do a rating until a Norwegian told me how long you last in cold water.
SGC