Movie Location?

I have no idea if they still exist.

My guess, off the top of my head, would be 'no', since the land there is valued quite highly by the super rich, a clique of people I have absolutely nothing to do with.

I would assume that there would be some mention of it, if it still existed.

But, I could be wrong, for all we know.

I'm pretty sure the little island with the bridge is Bay Island, which sits in Balboa Bay (also known as Newport Harbor). I don't know if the house still exists. You can't just drive onto Bay Island and cruise up and down the street looking for it. There are only a couple dozen homes on Bay Island and nowadays automobiles are prohibited. The residents use golf carts to get around the Island and to go back and forth on the bridge to the Peninsula side of the bay.
The Balboa Island Ferry, which Joan Bennett and James Mason ride back and forth, still exists almost exactly as it appears in the movie. There are still 3 ferries running, taking 3 cars each along with foot traffic and bicycles. The ferry runs continually, traversing the approximate 300 yards of the channel between the Balboa Peninsula and Balboa Island, which is the most populated Island in the bay.
As Joan Bennett drives her outboard on her early morning "mission" you can see the silhouette of the cupola of the Balboa Pavilion, which remains the same today as it did in the early 1920s. The bridge she drives the boat under is actually Pacific Coast Highway which crosses over the channel and separates the main Bay, where all the action is, from the Back Bay which was called the swamp in the movie.
I've lived a lot of my life in and around Newport Beach, and it's funny, I have never heard any local resident,long term or otherwise, mention this movie. I only discovered it myself a few weeks ago on YouTube. Go figure.