This guide will let you know how to make Water in Subnautica. You have to follow the simple steps in this guide to make water.
To make the water, you first have to find the Bladder fish and then catch it, as shown in the image below.
After that, you need to go back to your base, use the Fabricator, and then go to the sustenance. You will find the fishes you can cook, so you have to select the cooked peeper to cook it.
You will need to go to the water tab and select the water there and the bladder fish, which will craft a water bottle, as shown in the image below.