As you wander around the pristine decks of a brand-new cruise ship, you stumble upon a curious sight: a small, bright yellow rubber duck staring up at you from the bottom of a potted plant.

If you’ve been on a cruise in the last few years, you might have noticed some stowaways: little rubber ducks nestled in plants, on banisters and doorframes, and peeking out from hard-to-reach ...