Submerged habitats are very unlikely to be a thing. Though humans have lived ON water for a long time in large numbers. There are many people living in boats and barges, veritable citys in south asia, and other areas. Submerged structures will suffer from hugely expensive engineering and construction costs, and you dont have to go very deep at all, before the pressure differential is greater than in space. Salt, corrosion, maintenance, wildly complicated and expensive.Air handling will be very loud, and prone to failures, water and waste, will also be extra complicated, leaks will be profoundly unpleasant.If we are talking about high end resort or theme hotels, then , whatever. On the other hand Mega floating and semi submerged structures can be built from any and all materials, and the engineering is strait forward. Still there is a vast amount of barren and waste land ,that can be utilised at much less effort, so none of it makes sense, except situationaly.... Singapor, and such.