If you look, it's for powered craft from the weir to Farndon bridge. If you launch upstream then come down river:
A) they're unlikely to stop a small inflatable
B) they mainly patrol from the weir to the red house pub
C) wherever you launch, Plead ignorance and say there were no signs upriver of the Farndon bridge and say you will apply
D) how much is the fine? Doubt it's more than £50.
E) there's hardly ever a patrol boat
I'd just take a chance and don't speed!!!! If you see him. Stop engine and row!