When a Ship (Submarines, likewise) mention “Depth”, they talk about KEEL depth (distance from bottom of Vessel to the surface). Most Submarines (in today’s Navy) are 30 feet plus in diameter and draw (while on the surface) 27 feet plus (Tridents draw 36 feet or so). 50 feet (keel depth) is too shallow for MODERN DAY SUBS, OK for WWII subs (much smaller)
Could the Q drop refer to the older subs in the days of the Hunt For Red October? That’s sort of what I meant but I am ignorant and don’t know how to ask useful questions....