Do you want a single engine or multi engine ATP? Either will require 50 hours in class. Jack Brown Seaplanes in Florida cranks out a bunch of private and commercial SES ratings, I don't know if they do a SES ATP. And I don't know if they have a MES ATP program. There's really no reason for one since most SES jobs only require a commercial certificate. There is a Twin Beech on floats in Lake Havasu AZ that does the multi rating. I believe the Seaplane Pilot Association has a listing of seaplane training schools on their web site. I'm stuck behind the Great Firewall for a couple days so it's a pain to track info down. Most SES jobs are in Alaska. There was a recent article in the SPA magizine about an airline in Puerto Rico that was having a hard time getting pilots because they operate Twin Otters on floats under 121. There aren't that many pilots with 50 hours MES looking for a co-pilot job. And remember you will have to complete a ATP CTP for the MES ATP. None of the seaplane schools are approved to conduct that training.