My advice to anyone thinking of a Naval career, but worried about sea-sickness ,is to try and serve either in a submarine, or a large aircraft-carrier, if your chosen navy is fortunate enough to have them ( it is not an option in the Irish or Belgian fleets, for example ). Submarines,whilst a bit lively on the surface, can travel in relative tranquility at a reasonable depth (or so I have heard ) whilst large carriers ( large as in 45,000tons + , Eagle, Ark Royal, from the 1960s ) need to be in pretty severe weather indeed before the motion becomes unpleasant.I travelled 112,000 miles aboard Eagle, between 1966-69,and only felt "queasy" twice --once,off the west coast of Africa, and once ' en route' from the Arctic to Edinburgh !!
Later on, I did some trips in the sail training yacht ,HMSTY Merlin, between Portsmouth /Cherbourg/Alderney/Poole, and mustered my dish & fed the fish every time !!