That is a most interesting 'photo: it must have been put there at least 68 years ago and it is remarkable how it has survived and is still quite legible. (Assuming it to be a recent post). Ones I have seen were usually white with clack lettering but of course this may have varied from Region to Region or administration. I am curious to the use of the building, when the sign was painted, and another point of interest is the wall, on which the sign is painted, as it seems to be a bricked in doorway.
I presume it make reference to the canal which was close by as open water - as opposed to hydrants - were always clearly marked. Static water tanks were also located in town and ares of high risk where open water, such as a river or canal, was not available. Tidal water was part of the knowledge that firefighters needed to know but also knowing the times of high and low water was essential.
In the watchroom where I was a firefighter we have a list of all open water supplies, including tidal, which had been pasted there during WW2 when the station was that of the NFS. Sadly modernisation removed all the older paraphernalia and this list now only serves in memories. I have to add that during the drought of 1976 this list was most useful and I, with one or two others, checked that these sources of water still existed. Fortunately most still did.