The trunks of trees evolved to support the weight of the tree as it grew taller and taller to compete with other plants for sunlight. Plants that live in water have a density very nearly that of water itself so if they grow large they are supported by their own buoyancy. Also trunks are very rigid. They stand up well to the forces that air currants can produce but they might not do well against strong currents of denser water. Water plants need to be flexible.