The waterfill mod is overpowered since you can cut off enemy bases by building water around them. Of course there are enemy swimming mods but then the water has no defense sense at all anymore.

What do you think of an option allowing waterfill only up to 5 tiles away from original water tiles?

It wont be an option im afraid, its down to each player to decide how cheaty they want to be or not.
The technical side thou is how do i tell what is original water and mod placed water as its all water and if i could then tell the difference then counting how many tiles away i am really isnt going to be ups friendly.

Thank you for your answer! Thus I will try it on my own. Is there no way to check if a tile is a water tile according to initial chunk generation?

with my limited coding knowledge no way that i know of to tell the difference.

After rethinking the problem the solution seems to be quite easy. I just have to check if its currently shallow water, deep water, or land fill. If its any of these three tiles at the moment, it is a "original water" tile.

And if someone wants to place a new water tile there has to be shallow water, deep water or land fill tile within 5 tiles radius.

This way the waterfill mod cannot be misused anymore to cut off whole island areas with enemies for easily taking over the ressource rich part of an island. But still you can enjoy landscaping (in a limited way) to edit shore lines for example.

Technically the function count_tiles_filtered{} looking for "water-shallow", "landfill", "deepwater" with radius 5.0 should by > 1 to allow placement of waterfill.

I found a way to make this work, borrowing from the method used in the CanalBuilder mod by placing dummy tiles and then replacing them if the placement is valid. I'm not constraining it to original water tiles but to any new water tiles as well. Check it out and feel free to steal it.

I found a way to make this work, borrowing from the method used in the CanalBuilder mod by placing dummy tiles and then replacing them if the placement is valid. I'm not constraining it to original water tiles but to any new water tiles as well. Check it out and feel free to steal it.

https://mods.factorio.com/mod/Canal

Instead of using any waterfill mod I now switched to using this mod https://mods.factorio.com/mod/Landfill_plus which allows to do landfill which can be erased afterwards. That way you can correct the shorelines by adding landfill. And if you do a false click you can erase the landfill to have water again. That way no waterfill mod is needed.