Some substitutions for daily phrases will go a long way. Head for toilet, for instance, referring to left and right as port/starboard, front as forward and back as aft, deck instead of floor. "Swinging the lead" for slacking off. Those are just some things off the top of my head--doing a Google search will yield some nautical phrase glossaries that you can use too, but little things like that slipped into normal conversation will give the idea that your character's a sailor without having them sound like they should be wearing a bandanna and eyepatch, particularly since you say they've been ashore for a while.

Stuff I'd avoid: "cabin" instead of room, "bulkhead" for wall, avast, matey, only do the port/starboard thing occasionally... stuff like that would probably not be easily understood by non-sailors and therefore more quickly washed out of a landbound sailor's vocabulary.

Another thing that isn't jargon but would also help is for your character to always be aware of the weather (regardless of era) since that's absolutely crucial for sailors. (If there is actual weather in your game.)