> Both sides weigh equal amounts so there's no net force.

Water at 40m depth has a different pressure than at the surface level. Pumping at the surface and releasing at depth will have significant water pressure difference.

(A quick Google later). At 40m, it's 5atm or roughly 75psi pressure.

(Another quick Google). Oil pipelines run at a higher order of magnitude as that, so it's doable but would return energy, increasing as you go deeper.