Saltwater is bad, but sediments will protect most of it (hopefully). In general way worse, than on land I would say.
Ideally, something is conserved under dry and oxygenfree conditions. But even in a swamp, without oxygen - amazing things have been found.
The question is probably, was everything covered before the salt water came, or afterwards. And stable. And while in the water with currents that can change, it is possible that everything gets washed free and then covered again over time. Also not good.
(Also exicted about the topic, I am strongly thinking about buying a professional metal detector. Close to where I live, there was one 3000 year old burial site found, I strongly suspect, there are more. And this site, is also not that far from me and 20 m is something you can dive (but I need to train that again))
>Ideally, something is conserved under dry and oxygenfree conditions.
As a result, a disproportionate amount of our knowledge of Roman bureaucracy comes from Egypt, simply because papyrus preserves so well (even accidentally) in so much of the area. It makes research somewhat awkward, because we know Egypt received special treatment in some ways but it's so often a sample-size of one that we don't know if lots of things are an Egypt thing or a Roman-province thing.
I'm guessing the "special treatment" was that they were nerfed. https://en.wikipedia.org/wiki/Roman_Egypt "Egypt was unique in that its garrison was commanded by the praefectus Aegypti, an official of the equestrian order, rather than, as in other provinces, a governor of the senatorial class." They didn't want another Cleopatra/Mark Antony situation.
Also drag-net fishing and boat anchors are really, really bad to preservation of sites. Unfortunately most of the likely archeological sites are also, for obvious reasons, just off the coast where such activities are common.
One immediate benefit is that there would've been fewer thieves looting the artifacts.