This move does not consume the user's Water Gem, and cannot be redirected by the Storm Drain Ability.
This was the reason it wasn't redirected. On Turn 1, Water Pledge was targeted to M-Camerupt- this was why Storm Drain activated. However, on Turn 4 it was targeted to M-Gallade, not M-Camerupt. Because of this, Water Pledge didn't caused Storm Drain to activate.
Hope this helps!