The sailor admitted unauthorised physical contact and unseemly horseplay causing embarrassment.
Prosecutors accepted his not guilty pleas to sexual assault and assault at the court martial at HMS Nelson, Queen Street, Portsmouth.
The court heard how the sailor, a father of two, was celebrating his birthday and grabbed the 21-year-old Wren by the ankles and thrust his body against her.
Fellow sailors said they laughed at the action.
One said: ‘He lifted her legs and slid her down the seat so her calves were on his chest.’
Another said: ‘The Wren herself said she thought there was nothing malicious about what happened, just that it was in poor taste.
‘He is a reputable seaman who served in the Gulf.’