Many of the fish, invertebrate, and marine mammal stocks will never recover to the levels they once were at before the industrial-scale impacts we’ve had, even if humans and all our current impacts on the seas could be pulled out like skimming toys out of the tub. Many species and populations have become functionally extirpated and/or displaced by other species more resilient to our impacts.
That’s not to say overall aquatic productivity and species abundance wouldn’t go up and get to similar biomass levels previously seen, but the species structure would certainly be different as we’ve had huge impacts on genetics of various populations, and recovery would look different in different areas.
Imagine a wheat field being left alone for the next hundred years; no tilling or re-seeding. It will be green again next summer, but certainly not with the native mature prairie species it had for thousands of years prior. It would take a long time to restore that kind of environment in the middle of a grain farming area. And some of the agronomic species we’ve introduced are so competitive with native species that they can permanently out-compete the successional species that naturally transform a landscape. I’ve seen photos of a simple cutline planted to grass in the middle of forests where the grass persists for decades with little to no sign of trees returning. Oceans and mobile species can be far more impacted by change.
So, in some areas some marine stocks may well recover to very high levels in 5-10 years, but other areas and populations recovery is hopeless and their place would be filled with other species which may explode to numbers never seen before, which would lead to a bunch of up/down cycles before leveling out somewhere a century or two later… but whatever that composition looked like it’s unlikely to look the same it did before we started having major impact.