Coast Guard Island is located in the Alameda Estuary and is the site of a number of Coast Guard facilities, including Commander of the Pacific Area, Commander of United States Coast Guard Sector Eleven, and several large cutter-class ships.
Coast Guard Island is an artificial island that was created when the Alameda Estuary was dredged in 1913, extending the Oakland Estuary and connecting it to San Leandro Bay. Originally known as Government Island, it has been used by the Coast Guard since 1926. In 1931 the USCG received official title to a 15-acre plot and permission to establish a permanent base and improve infrastructure on the base. The base continued to grow and develop and by 1942 the Coast Guard had acquired the rights to develop the entire 67-acre island into an operational center and a training facility.
The island saw its heaviest use and quickest development during World War II when the training facility on the island was rapidly expanded to meet the nations increased need for service men. The training center opened in 1942 and had facilities for 900 recruits. Following the end of World War II the island remained an important Coast Guard training center. During the next few decades the training center in Alameda remained the largest Coast Guard field unit on the West Coast.
In 1982 the training center was closed and training facilities were moved to another base in New Jersey. Pacific Area Command moved from its site in downtown San Francisco and took up residence on Coast Guard Island.
Getting There By Bike...
I don't know that you're necessarily allowed to get onto Coast Guard Island unless you're a member of the Coast Guard. However, you can get near it and see the Coast Guard cutters that they have stationed there, from a variety of directions. If you ride across the Park Street Bridge on the west pathway you can see part of the island and the cutters that are moored in the Alameda Estuary. If you cross the bridge and start riding down Embarcadero towards Oakland, there is a bridge that crosses over to Coast Guard Island at the intersection of Embarcadero and Dennison. The intersection is just to the west of Union Point Park. If you're in Alameda proper, you can go to the end of any street between Grand Ave. and Schiller and see the island and the ships. The island is roughly parallel with these streets though to get a really good view you have to ride into the Marina.