Growing algae may be a viable way of getting energy. The yield/acre is extremely high compared to any other crop. Some algae contain large quantities of oil.
Harvesting algae is very expensive. Especially getting the algae out of the water, and the oil out of the algae requires an expensive centrifuge or substances like hexane. The reason algae are commercially grown is because of the other substances that come out of it; strains like Chlorella and Spirulina are being sold as food supplements and used in cosmetics and pharmacy.
There are two ways to grow them, open and closed systems. Open systems are cheaper and a lot simpler (almost all commercial growers use raceway ponds) but growing them at sea is difficult. You have almost no control on temperature, acidity etcetera, but worse is that contamination with other algae is more likely in windy situations with a lot of competing algae (and bacteria) in the air. This might be overcome by using a fresh water strain, but then the water gets expensive (and may be polluted with salt in no time). Closed systems are more expensive, and the major disadvantage: you'd have to import nutrients, (clean) C02, and methanol.