Three that I know of. Rick Hensch at Florida Seaplanes does the multi on a Grumman Widgeon if you want to get the amphibian experience. It's about $500/hr and takes about 5-8hrs to do. He'll let you do the MES alone (one of the few who will), but it would probably not be a bad idea to do the SES on his Maule on floats at the same time.
Florida Seaplanes
Sheble Aviation in Arizona does a multi on their old Beech 18 on floats on Lake Mead and the Colorado River. Very beautiful part of the world. You'll have to do their single engine rating first on the Lake Buckaneer LA4-200 before they allow you on the Beech.
Welcome to Sheble Aviation | shebleaviation.com
ESP Aviation in Florida also does a multi rating on the Republic Twin Bee which is another amphibian. Think you can do the MES alone.
Multi Engine Seaplane Rating | Twin Engine Republic Seabee Training in Sunny Florida
I researched extensively a couple of years ago when I was looking at buying a Piaggio Royal Gull (which is an amphibian). My plan was to do it on the Widgeon, as it was the closest I could come to the Gull and Rich seemed like a nice guy. In the end it didn't happen, but it's on my list of things to do at some point. A note - unlike in EASA land in FAA land you need to do a check ride for each rating. A MES check ride does not give you any privileges on SEL aircraft, bizarrely enough.
And when you're done, you can start saving up for the Gweduck kit!
