I have read a similar proposal that you can just drop biomass into deep oceanic trenches. The cold, pressure, etc mean that decomposition is significantly slower than would happen on the surface.
That model only works in an ocean without currents, or with exclusively favorable currents. The weaker claim of, "we can expend minimal energy by taking a very long time to get there," perhaps even wind energy, might work though. I think we'd have to answer that empirically.