Waterwalking isn't bad persay. But it pales substantially vs Gathering Storm or Scorch.
There's just not many scenarios where Waterwalking adheres to "Safe play" or even combat situation.
You basically have maybe 5-6 river objectives per game, and when in combat, the river is the LAST place you want to be, because it's open, and hard to get to a safety. The subjective nature of the rune just makes it less viable (even on junglers)
Furthermore, Gathering/Scorch almost always outscale it on damage, or raw stats. And movement speed has a soft cap after which investing severely in move speed becomes an exercise in futility.
I almost EXCLUSIVELY use phase rush/celerity/nimbus cloak and on a lot of champs, Magical Footwear. My MAGES have base move speeds of some 467 or more. Karma has 524 on me, and bursts clear up over 700+ Move speed sometimes. Riverwalking might briefly put me to 709 on that build before dropping down like the rest of the boosts. At some point, it's just better to take the damage over the move speed.
There's just better move speed options (The ones I mentioned, + Relentless Hunter)