The Sea of Gatan was a large body of water.

Miral and her daughter B'Elanna Torres once visited the Sea of Gatan. Young B'Elanna nearly drowned in the sea's deep water, but luckily Miral was able to resuscitate her. As this was B'Elanna's first brush with death, Miral told her about the Klingon afterlife, including stories of Kortar, Gre'thor, and Sto-vo-kor. (VOY: "Barge of the Dead")

According to deleted lines from the script, the Sea of Gatan was on Betazed, and that B'Elanna was, at the time, five years old.
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