You do not need to swim out in the ocean, the cache is not at the posted coordinates. You must complete the Adventure Lab: Hwy-1 Jenner to Salt Point to acquire the coordinates for this bonus cache.
Now, to continue with your history lesson!!
Russian Gulch (where you are standing) was another of the Doghole ports used in the late 1800s. It was supplied by the Russian River mills via a steam railroad system. Some of the roadbeds can still be found in the area. This was a later built port, its use dating to the late 1890s. At first it consisted of a slide chute but later converted to a wire chute extending to the bluff just south of the beach. The wire chute is connected to the Western Redwood Lumber Company. Using this more modern technology it was active after the turn of the 20th century. This area's archeological finds are slightly different from the other doghole ports. Here have been found more wooden remains: railroad ties, spaced timbers and wooden fragments.
There is no cell service at this spot therefore I included the history of this area in the write up.
Remember the entire Adventure Lab is in areas with LITTLE TO NO CELL SERVICE, it is the coast therefor the geofences were made extremely large.
This hide is best found in the southbound direction. It is a true HIWAY HIDE, exactly what you would expect on the hiway.
You can validate your puzzle solution with certitude.