Phil,
The (partial) fencing in of canals began in the early 50s after many lads had drowned year on year - the brother of a boy in our class at Clifton Rd was one as Fred can confirm - it wasn't a success so the Council sponsored a competition in schools to invent a slogan and or design a poster - the resulting campaign was centred on "CANALS ARE WATERWAYS NOT PLAY WAYS'', and this was more effective. Probably because it emphasised the dangers of canals rather than trying to prohibit boys from them altogether - that was always going to be open invitation to most lads.
tramp