Pilot DAR hit the nail on the head, and they are certainly the key issues. If you are a private user, with typical financial resources, then the cost of salt operation is likely to be prohibitive. Parts for seaplanes tend to be notoriously expensive... try an oleo for a LA-4 Lake @ $1950++ and you wont be so keen to rust them out in salt. A commercial operation will have a cash flow that may make the costs on maint/permissions/infrastructure viable. Perhaps the safest path to try salt water theories is one of these
Polaris Motor whcih according to C206 Pilot extraordinarie who owns both says the Polairis can manage a 5ft swell.....
If common sense prevails here is a few spots you can probably secure permission to use if past use is anything to go by:
53° 8.162'N 2° 5.167'W
51° 7.406'N 0° 28.456'W
50° 56.300'N 0° 51.733'E
There's probably more, but I don't operate in the UK so its not my bag. No real point in making inquiries if you don't have a real move on purchasing/operating a seaplane there, as it is quite easy to sound like a day dreamer. There is a guy operating a Kitfox off floats somewhere on the "mainland" big article on him in the LAA magazine. He might be best bet for accurate info on the job.
Irish