Any time I have been canoeing (not very often admittedly) I have just went to the water and done it, I figured its quicker to ask forgiveness than fart around getting permission.
This is an irresponsible attitude to take! There are many people working to get access for paddlers and if you just put your boat on the water as you feel like it, then you risk wrecking that work.
The odd fisherman (with the emphasis on the ODD bit) has raised a fairly vocal objection but what are they going to do, swim after me?
Well, for starters they'll throw 'kin great rocks! I've also met a kayak paddler who had damage from shot gun pellets in his boat from a particularly nasty incident in 'legally entitled to paddle where you like' Scotland. You mustn't forget that anglers pay vast sums of cash to fish some waters and folk paddling through can seriously upset them.
I don't know if Mr Dazzler would be pushing the limits of the British Canoe Union with an Umiak, but BCU membership gives value for money with it's license which covers many miles of waterway in the UK. Also third party insurance to, I think, £3,000,000. As has been said earlier, tidal waters are usable, but expect to pay some sort of fee which is usually reasonably priced.
Al