Why is USS Saratoga (CV3) nicknamed "Sara Maru"?
"Maru"(丸) is a term widely used to name ships in Japan.
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I was able to dig up one source with an explanation.
It relates to your information about that being a common suffix for Japanese ships. Bruce Gamble in Target: Rabul asserts that it was originally a name of derision applied (I'm assuming by crews of other ships) due to her knack for managing to be indisposed during major battles with the Japanese.
The first two years of World War II frequently found Saratoga in the wrong place at the wrong time. ...
... Saratoga spent much of summer 1943 moored in port. Because of the two torpedo hits, she had missed out on three epic carrier-versus-carrier battles: Coral Sea, Midway, and Santa Cruz. Saratoga thus became known derisively as "the Reluctant Dragon" and "the Pond Lily." Probably the meanest was "Sara Maru," which put her in league with the enemy.
As a point of clarification those "two torpedo hits" happened in separate incidents, and each separately caused her to have to be laid up in drydock for repairs.
The word "maru" means circle but in Japanese mythology, Hakudo Maru was a spirit who taught humans how to make ships so the word or name is not meant to be derisive at all