Welcome to the forum from the southern edge of the Lake District. We are lucky here in that wild camping is traditionally tolerated on the fells - not sure that the farmers or authorities in N.Wales are quite so tolerant, but if you head high and are discreet you are unlikely to have problems.
Water never a problem as my dog (Springer Spaniel) delights in drinking from the muddiest of puddles - never seems to do her any harm. Water filter a good idea though - I use a Sawyer Squeeze filter, which is brilliant, and is compatible with standard 'pop' bottles.