Getting a sea plane rating is far different from flying on wheels.
The actual flying once the airplane is in the air is not all that much different than flying an airplane on wheels.
However once in contact with the water it is another world all together and that is where proper instruction from someone who actually knows the subject through experience in the commercial world of sea plane flying comes in.
Finding an instructor who has experience beyond the puppy mill mentality of the average flight school is the real problem.
Be very careful about recommendations from pilots who have just completed XXXX's course and babble on and on about how wonderful their flight instruction was at XXXX's school......in many cases the instructor barley knows more than the student....their pay structure will explain why.