Only landlubbers and lifeguards could regard 15 to 20 miles offshore as the middle of the ocean.
I sail boats, and I grew up literally next to Newport Beach. I've lived full time on a sailboat. I'm not a landlubber.
No one refers to "the middle of the ocean" as the literal centroid point of a mass of water.
When someone falls off of a bay slip into the water they say "I fell into the ocean" , and that's a hell of a lot less extreme than swimming for ~9 hours and ending up so far from coast that you could by-chance run into Catalina Island , which is part of the Channel islands located in (and surrounded by..) the Pacific Ocean , away from the coast of California.
Here's a rule of thumb! Between coasts and in something demarcated as an ocean? You're in the middle of the ocean! Now, as with anything, there are varying degrees; but lest you bring up the literal language here let me remind you : no one has been to the middle of an ocean, and the water will still sink your boat and drown you even 15 feet out.