Shipmate
/ˈʃɪpmeɪt/
Definition 1 of 1
Pro Tip 1/3Comradery At Sea
Implies a strong bond formed between individuals who have worked together on a ship. 
After facing that storm together, Lucas became more than just a shipmate to me.
Pro Tip 2/3Exclusive Term
Specific to people who serve or have served together on a ship, not other professions. 
You wouldn't call a coworker from the office a shipmate.
Pro Tip 3/3Context Clues
Pay attention to context to know if 'shipmate' refers to past or present crew members. 
My old shipmate from the navy sent me a letter.