Any seaplane can take off from any water surface. Just set the takeoff point there. Same for landing.
In some maps there are even waterplane airfields set on water but this is not necessary.
From my previous experience of campaign building it is correct that a seaplane could take off from any water but when selected that waypoint will automatically move to the nearest icon, usually at the nearest land airfield, so you have to then move it far enough away before saving so it does not do that,
touch down with auto set the aircraft had to have a designated landing position. The landing airfield icons with waypoints can only be assigned by the map maker.
There are items in the FMB that do enable you to set touch down on water for land aircraft, these are a sort of stand in carrier deck but I forget what they are called.
Admittedly it is some time since I did much in that department but this is the same for any aircraft wheeled or seaplane, if they are not assigned to that particular icon that is visible on the map then any AI controlled aircraft will orbit the way point.
Sorry. if there is no landing assigned the aircraft will orbit.Many map makers forget to add seaplane landing areas to maps or when they do there are too few.
The above is primarily a concern for AI aircraft added in missions, if piloting the aircraft yourself you can of course land where you want, I am usually forced by events to land a bullet riddled aircraft far from home.
Wishing you all the very best, Pete.