Maybe there is competition,. but no-one owns a beach though, I say go for it, look in the lower sections where the water washes in or out the front of surf clubs and erosion zones which are by far the best spots. The life guards and surfers are always cool, they know you will remove sharp and dangerous bits of junk you come across. This is over Northern Beaches, hav'nt detected on the the "Dark side" of the city, lol.
Manly Beach is well patrolled, have a go there, mainly get hassled more by the curious than a nutter.