The reason to cover both of them together involves a bit of basic biology ... so please bare with me.
Your cell walls are basically a membrane made of fats. This membrane works very cleverly to allow nutrients and fluids to flow in and out of the cell in a controlled manner. The reason the membrane is made up of fats is that your cell contents is mainly cell plasma which is a liquid, and the fats make the membrane waterproof stopping the liquid contents of the cell spilling out rather than the flow being controlled by the membrane.
As your body is at least 60% water it's pretty important that your cell membranes are capable of holding water in the cells. This is why essential fats as so important for health, and in particular in keeping the body young and working properly. Without sufficient polyunsaturated fats (the kind that are more fluid and are used to make cell membranes) your cell membranes won't function as well as they should and won't be able to keep the cells full of liquid. Water is an essential input into numerous biochemical processes in the body rangeing from detoxification to fat burning, to regulating blood pressure, so without being able to hold enough water in your cells these functions will be impaired.
This also means that you need sufficient water intake - it's all well and good having a nice waterproof cell membrane, but if you don't drink enough water to keep your cell full of liquid it will become dehydrated and shrivel like a deflating balloon. This isn't just bad news in terms of cell function, but also shows up as visible ageing - wrinkles are caused by a combination of sugar hardening the fats in your cell walls and your cells shrinking from being not properly hydrated. Suffice to say both essential fats and water are key ingredients to avoid looking like an old leather bag!!
What does this mean in practice? Well these two are actually pretty easy to tick off the list:
Water intake for an adult is generally recommended at 1.5 - 2 litres daily, but if you're exercising alot you'll probably need more than that. Don't just drink it all in one go! Start with a glass on waking and spread it out during the day to keep your body hydrated at all times. If you're not used to drinking plain water start with diluted fruit juices and herbal teas.
Polyunsaturated fats are found in seeds, nuts, olives and oily fish. If you sprinkle nuts and seeds on your breakfast cereal/muesli, have some olive oil dressing on your salad at lunch and eat oily fish three times a week you'll be getting plenty.