Just to clear up this point (yeah, yeah, I know!):
the term 'water table' as applied to SxS double guns is seen to be strictly American from where I sit. In Australia, we have strong firearm and shooting influences from both the US and the UK. US is 'water table', UK is 'action flats'.

Check the index in Burrard Vol.1 for example, no 'water table'. Have a look in 'Lock, Stock & Barrel' by Adams & Braden, something of a definitive work on Brit guns these days: 'action flats'.

I do notice the word 'water table' beginning to creep into the British auction descriptions more and more these days, those guys certainly know where their next meal is coming from!