Beware - flying floats is DEEPLY addictive!
Spent the whole Summer of 2003 flying floats (J3) in NW Ontario, returned to Europe, sold up and moved here permanently in 2004! You have been warned, it really can be life-changing.
Back then, Transport Canada would issue a temporary (1 year) licence for a small fee to allow accredited foreign pilots to pilot C-registered aircraft. In my case (FAA PPL at the time) I got the FAA ASES rating and check-flight in the States then did the Canadian training on top - in Canada it took the form of an instructor log-book endorsement. I found the FAA training to be the bare minimum (5 hours) and only enough for basic puddle-jumping, whereas the Canadian training (8 hours) had more emphasis on bush-flying in more remote areas.
The easiest way would be to get the seaplane rating on your UK licence, as I understand ratings cannot be added to the Canadian 1-year licence per se.
For training providers, a/c rentals and general seaplane info, take out a sub to the Seaplane Pilots Association (SPA). Best $50 you will ever spend.
Have fun!