Tuners for over the road diesels overwrite the manufacturer's electronic programming with new information that can increase fueling and therefore horsepower. But at the very least you void your warranty and may harm your engine.
The same thing is true for marine diesels and more so since marine diesels are often operated at high power settings for hours at a time whereas an over the road diesel only operates at high power when accelerating for a few seconds.
And they do nothing for part load operations. Marine diesels are designed to operate as efficiently as possible at part load. Tuning won't help that.