It's not that drinking too much water can slow down weightloss, it's just that soemtimes, your body will hold on to the water, causing water retention, making you weigh more.
But also, not drinking enough water can cause water retention too, as the kidneys will hold on to as much water as it can get because it's not getting enough, again making you weigh more.
Drink at least 2 litres (4 pints), and no more than 4 litres (8 pints).