adverb
describes the location or state of being in the open ocean, away from land
preposition
indicates a location or position, in this case, being on the open ocean
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The ship encountered rough weather while at sea.
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I love the feeling of being out at sea, surrounded by nothing but water.
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We were totally lost at sea trying to find the party.
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She felt like she was at sea in the new job, unsure of which direction to take.