The cache is not at the posted coordinates.
This bonus cache is for those who have completed my "Beyond Alki: The Beaches of West West Seattle" Adventure Lab Cache and takes you to one last beach. To obtain the final coordinates here in Lincoln Park, you must complete all 5 stages of the Adventure Lab Cache. Look at the journal entries after you answer each question, and you will find the coordinates and hints you'll need to find this Bonus Cache.
There is an "FTF" geocoin inside for the first-to-find!
I hope you enjoyed exploring the "far side" of West Seattle. Happy Caching!