RE: De-tuning a turbo DD 6V92
The DD book will have the allowable range of injectors you can install to bring down the power , but it may nor save the cost of the new injectors.
DD, esp turboed engines must operate at about 60% power and over to have efficiency.
Sounds like you are looking for 50 hp from 350 hp engines, which will not be efficient if you down rate them to say 300 hp.
Either different engines or a different boat , or man up , make huge wakes and enjoy 1MPG .
I would operate at 1100rpm or at 1lb of boost as a minimum to ensure engine longevity.