They should do it somewhere shallow first, next to a large city surrounded by water where the land values are high. Requiring a boat for access would make it less feasible, so perhaps just a stairway and/or an elevator at the water’s edge instead. Put it just deep enough that it’s not visually distracting to passers–by, and shallow enough that the residents can still see the sun through skylights.

The renders with tropical ocean life right outside the windows are a fantasy; most of the sea bottom is undifferentiated mud.

Of course you could just build a marina and let people live in houseboats. You’d get the same population density with fewer of the problems and lower costs.