So I'm making a somewhat "hardcore" modpack, though not excessively hardcore, and one thing that really stood out to me is how unrealistic being able to swim up waterfalls is. Swimming in rivers or lakes makes sense, but not up water that is flowing down.
There are two ways I can think to achieve this:
1. Stop players from swimming if they're in a block of downward-flowing water.
2. Increase the strength that water pushes entities, increasing the danger of getting caught in any water flow, not just downward, and becoming unable to swim against the current.