Various placeable waters. Very configurable.
Providing the player with new tools or adjusting the game interface, without fundamentally changing gameplay.
Prevent placing of waterfill on entities so that it cannot be used to simply "delete" biters
You can use https://mods.factorio.com/mod/Noxys_Swimming to do that. With that mod placing water over biters will not delete them.
Version 0.4.2 no longer allows you to kill biters or any other entities thanks to a recent addition to Factorio.
