Currently there is no option to disable the infinite ocean, unless going into the plugin C++ code and changing it to not do anything with the ocean plane mesh if you select this in the combobox as choice for the infinite ocean system. It is just a matter of simple and careful changes. The next planned release which will receive a major overhaul will include this.
The above currently deal with the infinite ocean part, but for having the ocean into a sphere or using it on parts of an sphere is something I never tried and despite it might sound neat it will incur in several performance issues. The current infinite ocean system is all about to make a square plane with more triangles in the camera position (center) and less triangles moving far from center (the edges) to be scaled up to fit the camera fustrum, this just to keep the number of triangles the same no matter if far or near the water surface. With this explanation, you will realize that your ocean into your planet to look good, will need quite a good amount of triangles along its whole surface and only mountains (if any) at ground level will occlude extra triangles out of the scene. So, the only way to have this done with good performance is forgeting about the ocean at certain distance and placing a simple water material in a sphere (your planet) which is hidden while the camera is near the surface and when the camera is far about certain amount you will just hide the current sphere with ocean to make appear the sphere with the simple water material. This can be achieve with LODs if needed be (same mesh, but different materials each LOD).
In the current overhaul we will use Quatree to show the water surface, so it is great for near the surface, but far from it we want to have a different approach. We might even make a demo map for a planet like surface (meaning we will need to handle the gravity), so this is just something else to make the development even longer. Hopefully we will achieve that, but not in a short timeframe, since this project is a community effort where people contribute when they have spare time.