I think most of the oceans are a barren wasteland.
That said, as a shallow and, at least at first, nutrient rich part of the ocean it could be a flourishing part of it. But if it’s filled via underground pipe, it’ll be isolated from sources of ocean life.
I think most of the oceans are a barren wasteland.
This is as far as I know, incorrect. I'm not sure why you think it would be true. Oceans, aside from areas like the Dead Sea are some of the most vibrant and life filled ecosystems we have. Even at the most crushing depths far from the sun there are entire ecosystems on the ocean floor.
The GP is technically correct. By volume much of the ocean is practically a wasteland because of the oxygen minimum zone [1]. A few animals like vampire squid are adapted to this zone but for the most part, the vast majority of ocean life is found in the top and bottom most layers.
In my mind, these new seas would have a steady influx of salt water, but only sweet water would evaporate out of them. And since I don't see the mechanism by which salt would get sequestered or removed from these bassins, I'd also guest their salt concentration would rise quite quickly.
Of course, it might still be interesting to have a huge water surface in the area that effectively serves as a rain generator, but it might not be the same thriving ecosystem we imagine when we say "ocean".
That said, as a shallow and, at least at first, nutrient rich part of the ocean it could be a flourishing part of it. But if it’s filled via underground pipe, it’ll be isolated from sources of ocean life.