To be pedantic I believe the TC requirements are:
7 hours in seaplane of which 5 MUST be dual. 1 Hr MUST be solo.
Said 5 water landings/take offs.
There is no formal handling test which does leave you with a bit of a hole when you walk out on the dock with your first passenger, strange feeling. I don't suppose anyone is going to let you out solo until they are reasonably certain they are going to get their plane back in one piece. Different if it is your own plane though.
Interestingly any commercial pilot with 50 hours in type (rating type, not aircraft type, ) may act as instructor. No instructor's licence required.