Based there for over a decade, she served with the United States Pacific Fleet, training personnel and testing new submarine equipment.

R-18 departed Hawaii on 12 December 1930, transited the Panama Canal, and thence continued on to the United States East Coast for inactivation.

On 30 May 1942, a U.S. Navy OS2U-2 Kingfisher floatplane mistook her for a German U-boat and dropped a depth charge on her as she crash-dived in the Atlantic Ocean 50 nautical miles (93 km; 58 mi) bearing 50 degrees from Bermuda′s Mount Hill Lighthouse.

[1] In August 1942 she moved farther south and until December 1942 operated in a training capacity in the United States Virgin Islands and at Trinidad.

Then assigned with other R-class submarines to training duties for the remainder of World War II, R-18 returned to New London on 24 December 1942.