Hello, I just wanted to add some reassurance. It was a real struggle to get my son (now 14 months) to drink enough until recently. However, although he sometimes got constipated he seemed happy, ate well, slept through the night etc so people kept telling me he must have been getting enough. A couple of times he was ill he suddenly started drinking gallons - it was reassuring to see he would drink more when his body told him to. (I also noted that adult vary hugely in their fluid intake - for example I drink a couple of litres of water a day and feel ill if I don't, whilst my husband happily survives on 2 or 3 cups of black coffee with no ill effect - so maybe needs of toddlers differ too).
When he hit 13 months it was all change. He refused a bottle and we found a sippy cup he liked (after several different ones) and suddenly he was drinking large quantities of cows milk at regualr intervals. We also give him water with his lunch too. I know all babies are different but I think for him it was partly an independence issue, he likes to hold the handles and be in control. We took to leaving sippy cups of water on low tables and shelves for him to find and help himself to. Unfortunately, after being a dream eater he now has lost interest in food and fussiness and tantrums in the highchair are frequent! But hey, he is drinking!
Hope this reassures you to some extent. Its so easy to agonise over it and then the little so-and-so decides to solve the problem themselves, when they feel like it.