Are there any beachside resorts in California that are lushly planted--reminiscent of the Caribbean/rainforest?

Tough question, Gina. California does not have a tropical climate, it is warm but usually very dry here. That being said, there are many lushly planted resorts but they are California lush as opposed to tropical lush.

Perhaps the closest you might get is Laguna Beach, Montecito/South Santa Barbara, or perhaps Carmel. All three are lush, Laguna is warm, Carmel and SB are temperate, none are tropical. Best I can offer.

Gina, I know where you're coming from. I used to think California would offer the same until I moved here 5 years ago from Florida.
Nothing here is tropical or lush. It is a desert, and along the ocean it is cold. Not that it isn't nicely landscaped, but you better believe every drop of that water is irrigated!
So for a nice area along the ocean, yes, Laguna is nice, as is La Jolla, Coronado Island, and Santa Barbara. Carmel and Monterey are beautiful, but cold by most beach/oceanside standards. Think sweaters and windbreakers.
As for a more northerly destination, the Lost Whale Inn in Trinidad, CA is an absolutely fabulous,newer style B&B with lush forest surroundings. It's perched on a cliff over the ocean, with trails to walk down to see the seals, a bonfire pit and a jacuzzi (both oceanview) and amazing rooms and services. Also close to the redwoods. Again, sweaters.
My pick of them all in SoCal is Santa Barbara. Sweaters there too except in summer, but it is a fun but laid back town with lots of neat shopping, a very accessible ocean (which is not always the case in Cali) and plenty of parking. Very good food too.
Have fun.

It is rare that I totally agree with anyone here, but I totally agree with the first two replies.
California (SoCal) is arid-ish, very dry and brown.
And the beaches tend to be cool compared to Caribbean and SoFla beaches.
Nothing in SoCal fits the St. John type tropical description. But kkep in mind that many Caribbean islands aren't lush at all...Caymans, Bahamas, Aruba, Anguilla, etc are very dry)

We live in So Cal and we love the beaches here. I totally agree with the others, So Cal beaches are NOT lush and not tropical The water tends to be much colder than the Caribbean. Santa Barbara is a great weekend getaway. Check out Ventura and Oxnard beaches too. They are not as crowded.