Methane based fuels such as liquefied and renewable natural gas are attractive fuels for marine applications to reduce CO2, NOx, SOx, and PM emissions. To realize the full GHG reduction of using methane based fuels, the exhaust methane emission (so-called 'methane slip') must be mitigated. This work details the measurement of the air quality and methane emissions from natural gas fuelled vessels, and evaluates strategies for their reduction. Through the use of a novel methane sensor, modified engine calibrations, and improved vessel and engine operation, the GHG emissions of a coastal RORO ferry were reduced, providing air quality and GHG emissions benefits relative diesel operation. Strategies to further reduce emissions through the use of hybridization are also evaluated using an emission model.