Water is going to be one of your most important necessities for surviving in the world of Grounded. It's a fairly hard to come-by resource in the early parts of the game, so you will need to keep your eyes wide open to find it. However, once you know what to look for, it will become much easier to keep yourself hydrated and not on the verge of expiring all of the time.
To get Clean Water in Grounded, you will need to look for large Water Droplets that are hanging off of tall pieces of grass! Once you find one of these, hit the piece of grass that it is hanging from and it will be dropped onto the ground for you to drink! There's a bit more to know about water in the game, which I will go into detail about below.
As mentioned, you will be looking for the large drops of water that are hanging on the grass. Just keep your eyes looking upwards if you're trying to get hydrated. You don't need a tool to knockdown the water, you can just punch the grass and it should fall to the ground.
Once it drops down, head over to where you saw it fall and you will get the Slurp option available to you. This allows you to drink the water without being poisoned. Any water that is on the ground and it isn't moving can be drank, but it's not good for you and it will drastically drop your hunger stat. You can eat afterwards and be fine, but it's not the best method. However, if you have no other option then you might as well drink it and deal with the consequences later!
As you get further into the game, you will want to make yourself a Canteen. This requires three Grub Hides and 2 Woven Fiber to craft. You can use this to collect fallen water droplets that can then be saved for when you need them. Storing this water away for later is always a good idea, so make sure you create a Canteen as soon as you can!