Since you've received some links to longer answers from the indomitable u/thefourthmaninaboat, I might perhaps clarify your question a bit. I'm not sure where you're getting the figure of 1200 Allied warships present, unless that's counting all the landing craft used in the invasion. There were seven battleships, five cruisers, 17 light cruisers and 139 destroyers or other escort ships that participated in the invasion, as well as something north of 500 other ships (transports, patrol boats, monitors, etc)., but it's not like the Allies parked 1200 battleships off Normandy.

While not wishing to discourage further answers, u/thefourthmaninaboat has several excellent answers on the theme of naval bombardment and D-Day. Bombardment in general here and here; battleships specifically here; destroyers and small craft here; plus some discussion this thread.