This mod modifies vanilla's entity/fluid physics slightly so that fluids that are falling will push players and items downward. The strength of the force is tuned so that you just barely cannot swim upwards in a waterfall. You can still swim up them if you have the Dolphin's Grace effect, but it's difficult, you have to move side-to-side.
Why?
You are not a salmon. Also, in vanilla people don't use ladders because they're only slightly faster than swimming up a waterfall, and ladders are harder to build. So this mod effectively adds ladders back into the game.
How does this affect water elevators using soul sand or magma blocks?
It doesn't, they still work, since you have to use source blocks to make a bubble column anyway. Source blocks on top of source blocks are not considered "falling".
Is this going to run code on every player tick to check if they're in a falling fluid block?
No, it's implemented with a mixin that just adds 3 lines of code in the middle of the vanilla method Entity.updateFluidHeightAndDoFluidPushing. There will be no performance impact.
Is this compatible with other fluid physics mods?
Generally yes, because I'm only modifying entities, not fluids or fluid blocks.
Won't this let you drown mobs/players in a 3-block hole with a source block at the top?
Yes. There are already enough easy ways to kill mobs that I don't think this unbalances anything. It also makes sense, if you think of a fluid source block as being a source of fluid, rather than a lump of fluid, e.g. using Stationary Source Blocks.
The You Cannot Swim Up Waterfalls Team
