Water is one of the main aspects you have to take care of while playing Green Hell. Access to it isn't easy. This next page of the guide contains information that will tell you where you can find clean water.
The fastest way of obtaining water is to take it from a reservoir. However, all rivers are dirty, so drinking from them will lead to a parasitic disease. The water from one of the oasis is much safer to drink (the coordinates: 47W 14S, 51W 24S, and 45W 31S). This water is, however, unsafe, so it isn't 100% clean.
If you have a bowl, you can boil water. To do this, you have to fill the bowl with water and put it on the fire.
Over time, while exploring the island, you will also find equipment blueprints that will allow you to obtain water efficiently. Such items are water collector (51W 26S) and water filters (51W 19S ).