Is it safe to swim in this body of water?

Oh, and if it does get hot in October, is it normal to sunbathe on the beach during this month?

A little further south, maybe....www.seaviewinnatthebeach.com

It's a very busy working marina, meaning there are boats, some of them pretty big (charter boats for parties, not just personal craft) going in and out constantly. Aside from the danger of being hit, there also tends to be motor oil in the water in marinas.

And there is no "beach" w/sand in MDR. you have to go further south (venice beach, santa monica, etc.) or north for that.

Actually there is a beach in Marina Del Rey called Mothers Beach, near the Ritz Carlton Hotel. I believe it got its name because the water is calm for little kids to swim. However, you are swimming in a marina, meaning outboard engine fuel in the water.

Go north of Santa Monica or South Bay for the better beaches.

Do not swim at Mother's Beach! The water there has consistently been found to have high levels of pollution, and it's often closed. The County has been trying to figure out how to keep it clean, but there's just not enough tidal action. That's why the water is so calm...

If early October, you may get lucky with the weather. September is actually our hottest month.

P.S. Playa del Rey, Venice and Santa Monica are NORTH of MDR. El Segundo, Manhattan and Hermosa are SOUTH.

Oops. Playa del Rey is just SOUTH of MDR. (I was thinking of LAX...)

And in answer to your question, "Is it normal to sunbathe on the beach during this month?" ... There are people on the beach throughout the year except during the most inclement weather.