The boat may be really fast, that is debatable. You would have to believe, as many times as they have been caught outside the rules, no one knows how fast it would be if in a completely legal setup.
It's not like the motors are torn down after every race to make sure nobody did any combustion chamber work, modified the cam lobe profiles, used fuel additives, modified the head ports, did any work to the intake, etc, etc.
None of those are allowed. None of those things are tested.
In my opinion, they should not be part of the rules if they are not checked.
Off the top of my head, they have been caught with illegal gear ratios, illegal camshafts, illegal lower unit profiles, illegal ECU's, under weight by a lot, and illegal flywheels.