These systems will only work with common rail engines - most boats don't have these yet.
Most of these systems simply over-fuel the engine the expensive ones claim to remap the ECU or bypass the ECU with a "superior" mapping these appear to work on road vehicles - sometimes.
The problem with a boat is that to take advantage of any extra performance you would have to change the propeller - but to what?
My take on these devices is similar to the attitude of the engine making fraternity back in the 1970's of which I was part, back then many and various gadgets were marketed which if fitted would add horse power or improve fuel economy. We worked out that if we fitted all of them the fuel tank would overflow and the car would break the sound barrier - they were all tried and tested and were found to be snake oil. If it was that easy to improve an engine don't you think the manufacturer would have done it? the same goes for the majority of the these tuning chips for diesels. Fitting them means something will have to give, usually reliability and fuel consumption. In a car its not so bad if the engine fails not usually a life threatening situation, different in a boat.