False Colours — Judith Saxton

In the end, Frankie has decided to follow Trevor's advice -- to leave home. She goes to meet him at a port, where he's promised to find her work, and when she thinks she's found the ship that he's on, she goes aboard. But it's the wrong ship and before she can undo her mistake, she finds herself at sea on a fishing trawler headed for the Arctic.
Not only does Frankie have to endure the rough life on board, but, having been mistaken for the galley boy, she must maintain the pretence of being a man. And when the skipper discovers her secret, she finds that she is now in his power.