Tell AI to avoid edges, especially ones over water (this does not include checking whether a knock could drown yet). Also make flakes pick a new random dx/dy on respawn to further reduce patterns. Also disable a couple of weapons for the particularly dumb AI levels, and disable switching for the dumbest.
Spread the droplets around more so it looks less square (looked especially bad with piano). Slow down large splashes a bit for dramatic purposes, also taking low-g into account. Small splashes you can't really tell anyway.