There were thousands of occurrences in the American Civil War alone. Molly Bean is known to have served in the 47th North Carolina at Gettysburg but was not at all unique. There was also a Union woman who enlisted, and even disguised herself further as a black man to spy behind Confederate lines - Emma Edmonds.

I expect it would be far less likely at sea, there being even less privacy, but not impossible. At least one woman in the Napoleonic Prussian army lasted long enough to be promoted to sergeant before she was wounded and found out. I'm not sure Dorothy Lawrence should count, since she was a journalist, not a soldier. Women can't have actually served beyond the late 1800s as men simply because medical examinations eventually became far more stringent. (Elisha Hunt Rhodes described acting as a doctor's clerk during the enlistment of the 2nd Rhode Island; men simply paraded fully clothed past him and Rhodes quietly signaled the doctor if he recognized an applicant who was unfit. The doctor would then make up an excuse to reject him.)