On a side note. My grandparents lived in Daytona Beach during WWII. I remember hearing a lot of stories about the German U-Boats sinking our supply ships off the coast. They would come out of the gulf, around the southern tip of FL and head north to join the convoys. They had black outs every night to prevent the silhouette of the ship from being seen off shore by the Germans, however, they sank them on a regular basis. I heard lots of stories of the oil washing ashore with bodies, parts of bodies, personal effects and dead sea life from the depth charges our side used. The beaches recovered quickly after the war was over with no long term effects.
I don’t think they monitor the oceans and seas like they do the skies. Before we got into WWII my father was searching for U-boats along the southern coasts of North America.
We still had some of the old wooden coastal watchtowers around when I was a kid.