Consider a space opera sort of universe in which there exist communities of water-dwelling humanoids. These ocean people are derived from rootstock humans, descendants of genetically engineered human-variants who pioneered the colonisation of water worlds, or of Earth’s oceans. They may live their entire lives without setting foot on land or contacting the open air above the surface.
We set aside the technical complications of actually performing the handwavery of genetic engineering for the moment. This question concerns how it still appears rather difficult to justify the existence of the ocean people: in most cases, it tends to seem much easier to first construct closed environments, fill those with air, put usual humans in them and work from there.
What good reasons might there be for undertaking a project that would result in engineered humans adopted for survival underwater?
Potentially relevant constraint: despite the setting being futuristic in tone, introducing alternative history (perhaps some events that end up causing biotechnology to have developed comparatively faster, or cultural developments that change the way people evaluate different strategies of space colonisation?) is also an open option. Neither does the original goal of the project have to specifically be space colonisation; this question only asks for some combination of circumstances to exist at some point such that building a race of merfolk would be something that would reasonably be done.