Bottom slides up inside Top and is prevented from falling out under excess load by Lock. The inside of Top is a hollow air pocket, lowering the density of the overall vessel.

The base of Bottom is hollow to increase its buoyancy so it doesn’t fall all the way to Lock as soon as it’s put in water.

When the whole assembly is put into water, the tubes on the sides allow water to fill inside the top (allowing air to escape so water can enter), up until it reaches the top rim of Bottom. This allows the vessel to sink slightly rather than float entirely above water, increasing stability.

As weight is added to the vessel, Bottom will be pulled down but the four holes in the top of Top will allow air to enter into Top, expanding its air pocket and, therefore, decreasing overall density of the vessel even further.

…I hope it works 🙂