There are two forms of longevity. One is, how long will it hold a charge? And the other is, How many times can it be charged and discharged before losing capacity or failing?

The uncharged state is at pressure equivalence to the outside environment, so there's no reason not to think that it can last as long as concrete structures in water can last: decades to centuries. The generator parts will need more maintenance.

The charged state is empty, with the interior water pumped out, and holding it will be a mechanical and materials engineering problem. However, the normal call will be for a daily cycle to replace energy that would otherwise come from solar. Reasonable management should prevent any particular cell from being held at charge for more than a few days at a time.

Lithium chemistry cells have a limited lifespan in number of charge cycles; 600-1000 or so for full discharge, up to 10-15,000 for very light discharge cycles. The water storage system won't exhibit those problems unless they fail to filter the water adequately.

Tests have been performed for the the durability of concrete spheres underwater over ten year period and found that concrete under the sea behaves the same way as concrete on the land in same pressure.

In other words, very durable if you make it durable.