I do know that Pokémon Go will report "You're going too fast" when my phone is sitting on the cofee table just down to jitter in the GPS.
Not sure what useful data a speed app on a phone will give you on the canal system
Playing around with GPS location tracking for canalplan it often reported an accuracy of about 100 metres, which if you're on a canal boat is several boat lengths. 10m jitter seemed quite common, so that's you moving from one of the cabin to the other in a second or so. Can't remember if I logged the reported speed, will have a look this evening