I bet there is a bunch of plankton and underwater growth like kelp they could use to make bio-diesel. Maybe even use some of those really large red trees too.
They are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 metres (100 to 260 ft).
For example, kelp may not grow in some areas because those areas are especially hospitable to the "Invertebrates that eat kelp include snails and shellfish such as crabs, sea urchins, and abalone." [0] If some of the increased value of kelp were directed towards decreasing those species then it might become practical to grow kelp there.