If you've started researching cruises online, or you're reading books on the subject, you've probably learned that those "all inclusive prices" aren't quite as all-inclusive as you were hoping. Sure, your cabin, your basic meals, and the cruise itself are included, but you'll find yourself paying for quite a few extras.
But surely not the water, right?
If you don't mind drinking tap water, your water is free. It is potable, but it's common for the water to have a strong chlorine taste, so it may not appeal to you.
You may find bottled water in your cabin, but don't assume it's free. Just like with a mini fridge in a hotel, you'll probably be charged if you drink it. Likewise bottled water available elsewhere on the ship will likely come with a price tag, unless you're on a luxury cruise ship where such things are usually included in the (much) higher price.
You should be able to find a card somewhere that lists the price of water and other beverages.