The maritime industry is already the most efficient form of transportation in terms of shipping goods and conveying passengers. New regulations are in place, however, to further reduce fuel consumption and greenhouse gas emissions for marine vessels.
The answer is electrification and hybridization of propulsion systems. Hybrid ships and boats have a conventional (typically diesel) fossil fuel engine and an electric motor, but when the electric motor is in use, no diesel is burned nor greenhouse gasses released into the atmosphere.
Marine current energy systems suffer from low-frequency (long-term) and high-frequency (short-term) fluctuations. Ultracapacitors can solve frequency fluctuation issues by smoothing the fluctuations in a hybrid systems with batteries.