re: Why does it need 500' deep water?
Because at the beginning acceleration of the pipelauncher you want the pipelauncher the lowest it can be.
The main body is in this case 300' long. Plus below this would be the "bottom section"- say, 100' long. Next above the main section you have around 50' of structure- therefore a total length of 450'- and you would probably want a margin of 50'- if nothing else, the ocean floor is probably not perfectly uniform.