For VERY casual and inexpensive Italian, you could try Alejo's, on Washington and Lincoln (in the mini-mall). Personally, I like Alejo's more for take-out rather than eat-in, because it gets pretty crowded. It's very "red sauce" italian. I like their chopped salads alot, as well as the seafood marinara.
4002 Lincoln Blvd
Marina Del Rey, CA 90292
There's also Antica Pizzaria, in the mall where the movie theater is on Maxella. This place gets mixed reviews on chowhound, but they are one of the few places in LA that serves authentic neopolitan-style pizza.
13455 Maxella Avenue, Marina del Rey, CA 90292
2nd Floor Marina Villa Marketplace (same floor as movie theater)
As another poster noted, for more upscale and finer quality Italian, you'll have to head to Santa Monica or West Los Angeles. One of the best Italian places in town is on the border of SM/West LA, Valentino.
On Montana, in Santa Monica, I like Locanda Portofino, although be warned, it can be pricey.
1110 Montana Ave