No problem. Indeed this game has a lot of stuff to learn, which is one of the reasons it's such a great game. I can't remember how many times I've started a world over.
Yes you need to mark the water to be harvested as with other resources (any body of water will do, the nearer the better). If you have a well it will be automatically used by the water masters (without marking), but it will not produce much water when compared to harvesting water from. You can also build rain catchers which will be utilized the same way as wells. Water purifier is an important building since it can make clean (drinkable) water from dirty water.
This could be is the reason why your farm isn't producing, if you have assigned farmers and marked some wild food to be harvested. You don't need many water masters though, if your village is tightly built and the distances are not big.