I've never been to your part of the world - have you been sailing in your area? Google maps suggests that the Exeter canal would be too narrow, and the River Exe is either tidal or covered with moored small pleasure craft. Also see
Exmouth and River Exe (including Topsham and Exeter Canal) [Expanded View] - Channel, West: pilotage, charts, photos and marine business listings which mentions a 10 kt speed limit.
However there may be a way round this- you wont know unless you ask locally. There may be a wide / shallow angled slipway somewhere. Find some contacts locally and ask!
From
ICON Aircraft | FAQ: Water Operations
Can the A5 be operated in salt water?
Yes. There are no set limits on saltwater operation; however, ICON does recommend that following any operation in salt water, the A5 exterior be given a good freshwater rinse. In addition, the A5 should not be allowed to sit in salt water for prolonged periods due to saltwater’s corrosive nature.
So you'd need to get it out of the water and wash it down after use.
I hope you can find a way to do this!