Now, I placed some water that was down bellow where the terrain is almost flat. and when I decided to continue with adding water, I got some kind of water stairs.
I used plugin called Part To Terrain and it worked fine for flat parts, but when I rotate it I get those stairs.
I think that would be a bug with your plugin. It would probably be best to either try to do it by hand, or find a different plugin and see if that works.
There isn’t really other plugins, I will try to contact developer of the plugin and see if it is a bug or if plugin is limited, doing it by hand will take me some time but I will try
To understand the issue in question, you need to understand how Voxel terrain works. On Roblox, terrain is handled via voxels, which are 4x4x4 stud areas which connect to each other dynamically. As a result of this, it is exceedingly difficult to make a truly smooth area of water - the easiest thing to do is to try and hide the ‘stepping’ effect with scenery such as rocks and broken branches or other foliage debris. Alternatively, you can try to minimize the stepping effect by manually going through and smoothing the surface with the terrain smooth brush tool, although your success in using it may be limited.
