I was a Coxswain (and maybe again) of a Harbour Patrol RIB. All i needed to pull someone for speeding etc was my eyes and that of my crewman. We can happily take a person to court and prosecute with just that. If it is that serious then we would get a independent witness as well.
That said we have all done 6K in a 5K area, of course. But for enclosed Harbours (& i imagine rivers(though i have no experience) it is more of a wake and general awareness consideration. So if you are making a big wake at 5K perhaps 2K would be more appropriate than trying to Plane?
As someone has already said there is an entire ocean to 'speed' providing it can be deemed 'safe navigation'.
Plus there are several other reasons for speed limited areas like Swimming Areas etc!
Have Fun & Stay Safe!