This toy is a wind-up freight boat that carries a large volume and propels itself forward. The buoyancy will be very large, to allow the freighter to carry large amounts of mass without sinking. This toy is meant to be used in the bathtub, and allows children to have fun with fluid physics and learn about the basics of motion.
The main principle for this is Newton’s 2nd law. F = ma. The point is mainly seen when there is a lot of freight in the boat. The more mass the boat carries, the slower it moves through the water. With the same propeller, the boat is given the same thrust, F. With more mass, m, the acceleration should decrease to compensate.
The toy also includes several differing designs in propellers, so that different levels of thrust can be provided. This provides even more space for experimentation, allowing the child to mess with variables as they see fit.
Another principle seen is buoyancy. As more mass is added, the higher the water level goes. If the water ever rises above the deck line and rushes in, then the entire boat sinks.
Since this toy is supposed to be submerged in the bathtub, the outer material should stay stable in water, and not react to any shampoos or soaps. Once again, plastic is a great candidate. It cannot rust and it is naturally non-reactive. To avoid metal screws, the multiple parts of the toy will be heat-melded together. If water ever gets into the inner compartments, a drain hole should be located on the top to vacate the compartments.