Looking out of the window I'd say almost anywhere in the UK would be OK for a splashdown.
I flew a J3 Cub with floats at Jack Browns, Florida.
Absolutely wonderful. (even better with the door off)
After experiencing the Cub I gave up tricycles and crossed over to the dark side………taildraggers.
Wonderful experience, but was told you can'y hire Floats solo in the US because of insurance probs.
CC
This float plane looks fun
http://millenniumaircrew.users.btopenworld.com/fighteronfloats.jpg