i get something called 'wahoo' from asda (think they sell it on amazon too) and it's a small box of water flavouring, strawberry or lemon. it has some vitamins in it and 1 sachet is 5 cals, each box comes with 12 sachets.
about £2 a box, but i see it on offer in asda for £1 now and again.
but i do agree with the bottled water Vs. tap water, even brita filtered water isn't as good as bottled.