Where a seaplane can be landed depends on the type of aircraft. For instance, some seaplanes are float planes, and can only be landed on water, and not on land. On the other hand, an amphibious aircraft can land on both.
Whilst a seaplane is great for its ability to land on water, and take off on water, you cannot just land it anywhere. Where you land a seaplane legally will depend on the state in which you are in. In addition, you also cannot land in National Parks, or protected areas.
In the United States, you would have to abide by local laws and regulations to determine where it is legal to land a seaplane. In many states, you can land in most bodies of water, but you should examine the Water Landing Directory from the Seaplane Pilots Association. This will help you find suitable landing locations in popular seaplane areas: https://www.seaplanepilotsassociation.org/resources/water-landing-directory/ .