I have played with this a little, HHO will boost the output from an internal combustion engine a little by lighting off the fuel earlier and producing a cleaner more efficient fuel burn.
I wouldn`t touch it with a barge pole, unless you can afford to buy sheets of gold or platinum to use as electrodes. Marine grade stainless steel will only last a couple of weeks before the iron is all leached out and deposited in the bottom of your vessel in a nasty brown mess.
Bob