“The US Coast Guard has entered into a Cooperative Research and Development Agreement (CRADA) with British diesel engine innovator, Cox Powertrain. The CRADA will evaluate and test the advantages, disadvantages, required technology enhancements, performance, costs and other issues associated with diesel outboard engine technology.–“There a been a big swell of interest in diesel outboards since NATO introduced its single fuel policy, with the military and naval forces of member countries keen to phase out petrol-fuelled outboards in favour of cleaner diesel alternatives.”
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Obvious advantages if we do not need to deal with two different fuels, in addition to the additional range advantage. Reading the rest of the post, sounds like this is close to realization.
Chuck Hill's CG Blog 
Mercury and Yanmar seem to be exploring the diesel outboard market already. I don’t know if they are producing anything appropriate for larger cutter boats but there are other some commercial options already.
Cox has a new plant and they expect to start building “Cox Powertrain’s CXO300 diesel outboard engine which goes into production in Q2 2019.”…”will be the world’s highest power density diesel outboard engine”
https://www.marinelink.com/news/cox-powertrain-announces-new-engine-439701
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