98
It just seems like it would be a really cool thing to have gills and be able to populate the oceans in the same way we populate the land. We could have houses and shops and vehicles, andgo on walks/swims and just kind of live underwater.
Start a whole new second species of human here on earth maybe, Who knows?
Yes, but not very good ones, probably. We're endotherms (hot blooded), and as a result we burn like 3 times as much oxygen as a similarly-sized shark. You'd need a lot of gills.
The rest of our body is also not very well suited to being underwater long-term. If you're adding gills you might as well change our silhouette, eyes, hair and skin as well, but you might not look very human afterwards. Maybe you could manage something merperson-ish, with an extra-flexible neck for looking towards where you swim and gills all along the tail? Sanitation would also be a bit of a nightmare, because if there's a sewage leak you get to breathe it.
The other option is just to get really good at diving conventionally, maybe enhance ourselves to make that easier, and build dry "indoor" spaces underwater. The technology to do it at a basic level isn't new, but there just hasn't been much interest in living that way yet.