Oakland/Jack London Square is another station that has very nice beach access---although it is not an ocean beach, and it is not a sandy beach either. But at least some of the beach experience can be had there, very easily. For that matter, Martinez, California has some nice access to places where you can...see salt water up close. But again, not a classical beach experience.
In fact, I just looked and was surprised to find--the Coast Starlight has all of one stops between Los Angeles and Seattle where a traveller can get out and stroll down to the ocean.
(Santa Barbara, and possibly Oxnard, depending on what you consider a "stroll")