As several people have pointed out, Lake Sammamish is used for seaplane
training quite a bit, since it is very near to places like Kenmore Air.
Since you live in the Seattle area, I'd highly recommend taking a dual
instruction lesson at Kenmore. I was visiting Seattle a couple of weeks
ago and arranged for some dual instruction in a DHC-2 Beaver. The folks
at Kenmore were great, and the experience of flying a Beaver on floats
was outstanding.

Kenmore also has a couple of super cubs on floats that are normally
used for training, and they are probably a more appropriate first
seaplane to fly than a Beaver. Still I'd recommend the Beaver to anyone
willing to pay the hourly rate. In 1.1 hours we did 6 water takeoffs &
landings and I'd say about 3 1/2 were unassisted. Coming from flying
various Cessnas & Pipers, its really fun to fly a plane with a big
round Pratt & Whitney engine on the front that takes 6 gallons of oil.
No matter which plane you choose, flying a seaplane is definitely an
experience to add to your flying background.

Eric B