“Can’t imagine how deep one needs to go with a foundation to find something solid to build a skyscraper upon.”
you can’t ... which is why buildings and structures in such coastal areas are held up by driving in enough pilings deep enough that the friction of the soil/sand surrounding those pilings is what actually holds those structures from sinking ...
Manually searching and looking through references I see various estimates from 3 to 30 feet to limestone bedrock, with most areas being to the lower end of that. No need to rely on soil friction.