Why the 600 foot Depth for Rapture :
I watched the inital descent 10..18 fathoms, then the window was covered, I kept counting time and then you emerge on seabed on a ridge where you look across/down on Cityscape (and the elapsed time made it something like 400 feet down). From there you mostly move sideways and go by some buildings, descending a little more and finally enter up on the side of the Rapture Transit Authority building, with its numerous floors below that spot (and a seabed some indistict distance below).
A 600 foot depth is far enough down to be completely dark - without any sunlight ever reaching it. Deep enough to have tall buildings that at the top still are some ways below the surface (one iceberg, too close, would tear the tops off those skyscapers (surface-scrapers?) - even with Mount Ryan (volcano) blocking progress of most icebergs). The Lighthouse is perched at a shallow spot on the top of the Sea Mount (undersea mountain/volcano) which gives us the possible 600 foot depth for Rapture's buildings, instead of the 2000+ foot depth of Rapture's given location in the North Atlantic Ocean (see Ocean depth maps West of Iceland -- http://www.tacopshq.com/MBX/Globalthunder/Maps/Charts/Arctic-depthchart.GIF ).
Too much deeper and it becomes horrendously impractical engineering-wise (ie- "Six miles under the sea" - muttered by a deranged Splicer isnt any great evidence for the actual depth, and there are actually VERY few spots in the Ocean THAT deep). The thick reinforced concrete, and Ryanium windows a foot thick, pressure seals, and working submarines are all problematic enough at just 600 feet.