You have some easy choices here.
Sweet potato plant. Start it at home, take a sweet potato, stick toothpicks on the sides and put it in a glass of water. Leaves will come out and go up, and roots will go down. Once it gets started, take the toothpicks out, put the potato in a plastic bag, fill the bag with water, and twist tie it at the top so water wont spill out while sailing. The water will last for quite a while, so just refill it the next time you get back to the boat. The green vines will grow very nicely, and grow very long.Air plants.The kind you will often see at a state fair. It sort of looks like a small bits of limbs from a pine tree. It lives without dirt, and can lives a long time without water. Just soak it in water when you remember it, and every year, with luck, it will flower. I've taken a bunch of them and attach them to a chunk of driftwood using some hot glue. When I remember it, I just soak the whole driftwood in the sink for 1/2 hour. I have gone over a month without watering it, and it still flowers once in a while. Tomatoes.Of course, I have a live aboard friend who grows bush tomatoes on the boat. They strap it to a stanchion, and the rain waters it sometimes. The tomatoes will even stay on while under sail, if they are not too ripe.