At the risk of being even more pedantic - it is not a sign off but you must take a flight test with an FAA DPE or inspector and pass - Those training you cannot also conduct the flight test.
The flight test includes:
Normal, rough, glassy take offs and landings
Idle, step and plough taxi
Emergency procedures (Engine failures are fun in a seaplane !)
Stalls, steep turns, etc...(the weight of the floats adds a new dimension).
It was great fun, in Michigan you can land on any lake where it is not specifically prohibited and there are hundreds. We spend 2 days flying around at 500', though occasionally got up to the dizzy heights of 1300' for the transit between lakes. I think I did about 46 landings & takeoffs in 2 days and I'd highly recommend anyone doing the course.