I don't know of any Marriott/SPG hotels on the west coast that are on the beach but also have lots to walk to nearby.

I haven't been there since they renovated, but the JW Marriott Marco Island had a nice beach. There wasn't much to walk to, though, and I think you'd need a car likely.

The Ritz in Sarasota is walkable to downtown, though you have to cross a busy road (with a signal) to get downtown. That Ritz also has a shuttle to St. Armand's, which has tons of restaurants and shops that are within easy walking distance of each other. The shuttle also takes you to their beach club, which was quite nice if I recall. The downside is that you aren't right on the beach. However, the beach club has showers, changing facilities, and lockers.