I like the concept (and also the music and graphics), but as BirdCat Nate mentioned, once I've carried down a bunch of weight to the lower decks, I felt pretty safe and newly dropped cargo just didn't affect the boat anymore. Maybe if old cargo would "rot" after some time (possibly quicker when in contact with water) it would be more of a challenge since you'd have to constantly replace old crates with new one. Although rotting anvils don't sound too realistic :)
Also (and I might only be writing this since I've recently played Unrailed and Overcooked), it would also be awesome if crates of food would have to be delivered to a canteen, or gunpowder crates / cannonballs to higher decks (in an attack scenario), so the player would need to balance the ship [and] deliver required supplies to the right places as well (sometimes removing them from the lower decks so they will need to be replaced with something else to maintain balance)... but I might be overthinking it :)