Los Angeles area beaches that come to mind are:
* Huntington Beach (especially across from the Hyatt Resort)
* Santa Monica (around the pier)
* Malibu
* Laguna Beach
...the closest to Marina del Ray would be Venice Beach...but I must tell you, ironically Southern California to me has never been much about the beaches...there's toom much other stuff to do and see that you don't get anywhere else.
If you'd like ideas - let me know - happy to provide some - and unless you have a reason to be in Marina del Ray - I wouldn't necessarily go there.