I would recommend Jack Browns Sea Plane Base in Florida, first class Instruction, well maintained fleet of Cub float planes and reasonable price. They are on the edge of Winterhaven Airfield and you could get some land plane time at Tailwheels.
Once you have the rating on your FAA ticket then to add it to the JAA ticket, you need to take the Seamanship Exam and send a copy of your FAA ticket and logbook to the CAA who will add the rating for the usual fee. No flight test or flight training required in the UK to do this. You can take the private Seamanship Exam at Ontrack Aviation at Wellesbourne with Alan Newton or the Professional Exam at Gatwick. Alan does a terrific one day theory course prior to sitting the exam and covers everything you need for private and pro. I did the private exam as a tester for going to Gatwick without problem.
To renew the rating you do it as part of the SEP renewal as long as you meet the SEP (Sea) requirements as well. All in LASORS. I renew mine by flying in Florida and revalidating by experience.
PM me for furher details.