Following repairs at Espiritu Santo, "Aquarius" returned to Guadalcanal to conduct rehearsals with the 3d Marine Division for the projected assault on the Marianas. On 4 June, she departed Guadalcanal with TF 53 and proceeded to Kwajalein, the staging base for the operation. This force had orders to act as a floating reserve during the Saipan landings and, when directed, to invade Guam. TF 53 sortied from Kwajalein on the 12th. However, the engagement with the Japanese Mobile Fleet in the Battle of the Philippine Sea and the unexpectedly fierce resistance of the Japanese garrison on Saipan caused the invasion of Guam to be postponed. After standing-by for over a fortnight, "Aquarius" and the other ships for TF 53 put into Eniwetok on 28 June to await further orders.