UPDATED
This post lists some findings about RW "seas", i.e. sizeable extents of salted water and "lakes", i.e. sizeable extents of non-salted water. I surveyed several different worlds created with the last version and I found the following:
A) General
1) Seas, lakes, as well as other geographic features like mountain chains, have slots, i.e. modular areas which can be occupied by such a feature or not. It seems that the decision to fill a slot with a feature is made randomly, depending on the world seed, when the world is created or the area is first visited.
2) Slots follow a module of 1024 x 1024 blocks. Some features use single slots, some use clusters of 2x2 slots (i.e. 2048 x 2048 blocks).
B) Seas
1) Seas take double slots, i.e. cluster of 4 slots in a 2x2 group and then occupy 2048 x 2048 blocks.
2) Of this 2048x2048 blocks, the actual water portion occupy approx. the 1800x1800 central part, the remaining approx. 124 blocks of frame is taken half by the sand border and half by the surrounding green.
3) From the above, it follows that seas may occur every 2048 blocks along either axis, at the minimum:
4) As said above, not every sea-slot is actually occupied by a sea. As far as I can tell, approximately only 1/5 of the slots are actually occupied by a sea; the other contains land of some kind.
5) All seas of a world are identical; but the sea model is different from world to world. Vegetation and other local details, however, may be different from sea to sea within the same world (for instance, trees and grass may be different).
C) Lakes
1) Lakes occupy single slots, i.e. 1024 x 1024 block areas.
2) Of this 1024x1024 area, the water part occupies approx. 600~800 blocks in the centre, the rest being regular terrain.
3) Therefore, lakes may occur every 1024 blocks along either axis, at the minimum:
4) Again, not every lake slot is actually occupied by a lake; as far as I can tell, approx. only 1/15 of the slots are occupied by a lake.
5) Lakes are not all identical; however they are not all different either. It seems like a small number of 'lake models' are generated for each world (2? 3? more?) and reused (randomly?) for all the lakes.
Hoping this might of some use to somebody, M.