Watering needs depend on your growing site's location and the genetics of your crop. If you have a fairly humble-sized crop, then various hand-watering methods should suffice. Some marijuana growers haul water out to their site and then use watering pots or run hoses from a large container to their site. If you have been able to find a site that is fairly close to water, and you have a small number of plants, it may make the most sense to refill from the source and carry the water back and forth. This limits your liability since you'll never be questioned as to why you are bringing 100 gallons of water into the woods.
If you have a lot marijuana plants you may have to consider more elaborate methods, such as using battery- or gas-powered pumps. These have drawbacks in that they are usually very loud and the sound of a motor can carry for milks: not ideal for stealth operations. These systems require a source of power and constant water supply and sometimes need to be set up in an intricate way involving many subsections like tubing and platforms. Once in place, they have the ability to deliver tremendous volumes of water, according to the exact schedule that you need.
Another method is to fashion a slow-drip irrigation system. Using a one-gallon milk jug, poke a couple of pinholes into the bottom, which allows a steady, but small amount of water to be released. The advantage to this is that you only need refill the jugs, which, once in place, will water your plants on a consistent basis. The marijuana plant never suffers from too much watering as can happen if you travel to your site only occasionally. If your plants aren't getting enough water, then either add more jugs to water specific plants, or make the holes larger if the problem is the water retention abilities of the soil. Of course if you're just growing a few marijuana plants, and they are on your property, using an ordinary garden hose works great.
One very cheap and readily available source of water is of course, rainwater. Clean rainwater is a good source for your plants because tap water can contain unwanted chemicals. A lot of water systems use chemicals to clean the water to make it potable, or drinkable. They can also add additional chemicals to neutralize the taste, so that you don't taste the chemicals, just the water. Tap water PH may also be above or below 7. While this is great for humans and our finicky taste buds, it can be detrimental to the plants. One thing to watch out for is the amount of sodium (NaCl or salt) in the water. If you water your plants with overly salty water, their eventual size could be reduced by as much as 50%. Tap water can also contain chlorides, sulfates, and other solids, which helps to explain why the sales of water filtration systems are so brisk. It is also why using distilled water is recommended for young seedlings, before they are transplanted.
If you are concerned about the quality of water you are delivering to your plants (or drinking!) there are test kits available for sale at gardening and hardware stores. They will help you deter- mine water quality and let you know which chemicals are prevalent in the largest amounts.
If you don't want to deal with this at all, then rainwater is an easy solution. The best way to catch rainwater is to place a large container under a drain spout during the rainiest months of the year. Usually this will happen in the spring so forethought is once again crucial to success. If the area being drained is generally clear of water-soluble chemicals, the rainwater should be perfect for the plants; Then your only problem is storing the water and transporting it to the site.