"Thornback" stood out of New London, Connecticut, on 20 March 1945 bound, via the Panama Canal, for the Hawaiian Islands. She arrived at Pearl Harbor on 25 May and conducted training in Hawaiian waters prior to getting underway on 11 June for the western Pacific. As she stood down the Pearl Harbor channel, a formation of Landing Craft Infantry (LCIs), running down the wrong side of the channel, forced "Thornback" to crowd dangerously near the extreme edge of the channel. In the process, the submarine damaged her sound dome, necessitating repairs and a two-day delay in departing.