I think you would have to use a vessel that could withstand underwater pressures. I imagine that would be on the expensive side, probably more expensive than the concrete blocks. And if the the vessel compresses at all, I think you loose efficiency due to the loss of buoyancy.
On a more general note, I think the issue with these novel storage methods is that even their more optimistic $/kWh targets are barely competitive with existing battery prices. And due to manufacturing scale, battery prices are expected to continue to decline. A similar thing happened in the solar market. Prices were really high, which led to a bunch of startups attempting to bring novel technologies to market (Solyndra was one of them), but once China started flexing its manufacturing muscle, PV prices dropped and the novel technologies had no hope of competing.
On a more general note, I think the issue with these novel storage methods is that even their more optimistic $/kWh targets are barely competitive with existing battery prices. And due to manufacturing scale, battery prices are expected to continue to decline. A similar thing happened in the solar market. Prices were really high, which led to a bunch of startups attempting to bring novel technologies to market (Solyndra was one of them), but once China started flexing its manufacturing muscle, PV prices dropped and the novel technologies had no hope of competing.