Gula Udden is accessible in many ways. A hot summer day, we walked from Styrsö Skäret and decided the best way was to wade through the water, thus skipping a lot of bushy terrain on land. Then we climbed the mountain, placed the cache, and enjoyed an incredible sight.
Public transportation: www.vasttrafik.se, search for Styrsö Skäret (optionally Donsö or Styrsö Bratten).
Consider asking the ferry crew for information about signal/semaphore usage; At some times, the ferry may not go to remote harbors like Styrsö Skäret, unless the semaphore is signaled or there are people about to go ashore.