There are three cases:
1. Seaplane landing expressly forbidden
2. Seaplane landing permitted (sometimes with restrictions)
3. Seaplane landing neither forbidden nor permitted, in which case
Common Law prevails (what is commonly done). If it is commonly done,
then it is ok. You don't need a law that says you can walk on a
sidewalk, people commonly walk there, you don't need a law, you have a
right by common law.

Each lake has it's own set of rules. All depends on the agency that
manages the lake, the neighbors, the laws of the state etc.