I must admit I like that idea better. The one thing you need to ensure the health of wood is a good flow of air and it does sound like the OP's plans would mean that it would be difficult to arrange this.

If I were going to use a timber solution I'd look at iroko or elm, which stands wet conditions well. Oak would be fine as long as it is well sealed from the steel, as it is acidic and will promote corrosion otherwise. Don't think I like the idea of softwood unless it's naturally resistant/durable like pitch pine (very hard to get and not easy to work) or larch (tends to split.)

Not convinced by pressure-treating-- fine for houses, not for boats.