Just to elaborate: due to wave action and leverage on the underside of the hull, parts of the deck of a ship can accelerate downward faster than the acceleration from Earth's gravity, and while the deck underneath the rocket has fallen faster than the rocket and its heavy engines it can also lurch to a side. Even if the center of gravity makes it equivalent to a sheet of rolled steel, so that it wouldn't tip over, it could become displaced, then go overboard or cause unwanted damage.

While I suppose that the rocket could be heavy enough to lower the center of gravity of the entire vessel below the troughs of any waves enough to make it incredibly unlikely, it isn't just the prospect of tipping over that makes it important to secure cargo.