A material doesn't have to be less dense than water to be viable for a hull. All ships sink if they take in water after all. If they were all made of materials less dense than water, at the very worst they would float poorly after taking in water and/or enough hull damage.
Ships float because the volume of water they displace is heavier than them. In a sense, the ship and its contents as a whole are less dense than water, even if the ship's structure is made of something denser than water. In layman terms, a ship's hull may be heavy, but it is hollow and the things filling it up (including air) are in average lighter than water.
That is not all, the hull needs a proper shape to make the most of the water pushing it upwards. I won't get into hydrodynamics here though.
Your hull must be strong enough to:
- Support its own weight and cargo;
- Keep its own shape under a lot of pressure.
We will probably never know the structural properties of prototaxites because they are extinct. But there is a company called The Living that is making bricks out of mycelium (the structural stuff of fungi):
If bioengineering can be used to make bricks, I don't see why it could not be used to make hull boards. We just have to tweak with the mycelium's structural properties until its tenacity, hardness, strength and maleability are close to that of wood.
In fact, if we ever manage that, fungal boats might become a thing. Fungi can be made to grow into arbitrary shapes much more easily than plants. They also feed on practically anything that has once been alive and don't require sunlight. Who knows, they might be cheaper and better than wood if we ever developed the technology.