Mermen

There might come a day when humans may have to consider engineering specific traits in their descendants in order to survive alien environments.
One such alien environment that might need adapting to is living underwater.
Several species of land animals on our planet have even gone aquatic at some time in the past but they all have something we lack, er I should say they all lack something we have, pores.

If humans were to try and live continuously in water we would bloat and die because we aren't water tight but there is one person alive who does lack pores... nipples and most of his teeth as well but that's beside the point [link].
It is possible that this trait could be engineered so that a race of people could become semi aquatic.

This is where it gets weird and might very well not work.

Removed by c section prematurely, the babies are connected by umbilicus to a device which simulates a human placenta, sort of a miniature dialysis machine/ intravenous oxygenator weight belt for buoyancy control.
If a baby never takes their first breath, do they need to? Perhaps over time our appendixes may regrow a way to extract oxygen from the water without assistance.

Until that time though I think that some of pore-less buddies' DNA should be saved if for no other reason than to discover how it is even possible.