I tried to make some plant model to upgrade the temperate biome. Placing them on mountains, grass is not a problem. But how it works when i want to place some models also in water?
That json alone will do nothing because there is no code to read it. You need the lua files that will search that code and do the spawning, it is not a vanilla thing, that was something I did for the modded biomes.
To spawn around water you can simple raise the water value for that category.
"mean_offset": {
"boundary": -5,
"water": 50
},
So objects will have a higher chance to spawn there (boundary is near terrain changing elevation)