The common trait between ships and that ducky inner tube floating in your pool is that both are governed by the scientific principle of buoyancy. That being the case, why not build cruise ships and the like from the tube’s same rubbery material? It’d certainly be cost effective, but it sounds better in theory than in practice. Just imagine what would happen if the ship got snagged by a rock and ripped. The sudden release of air would turn your Carnival Cruise into a hydrofoil nightmare, both it and you screaming across the ocean's surface at neck-snapping speed.