AFAIAA the water spray has nowt to do with foot and mouth - and it was around well before the last outbreak. I think it's there to give a quick blast to the side of arriving coaches to wash off a little bit of the inevitable muck and dust they accrue, which benefits the coach station environment (with many arriving coaches then later moving across to the departures side), with perhaps a little extra benefit of de-muckifying the coaches a little bit before they then head back out onto the London streets en route out of town.

I've also come across some suggestion that the water spray exists to dissuade people from walking along the route where coaches drive into VCS*, which would be a safety hazard - whilst that may be a secondary benefit, I don't really buy it as the primary reason (not least because there's no such water spray arrangement where coaches drive into the departures side of the coach station).

* VCS being Victoria Coach Station, for those who like slightly geeky abbreviations!
One thing that is worth noting about VCS is that its left-luggage facility is considerably cheaper than those at the mainline London railway stations, and is open to all comers, not just coach passengers.