To drink enough water is crucial for a healthy chameleon. The hard part is that a veiled chameleon can survive pretty well and pretty long with to little drinking water, but will develop many chronic diseases like kidney problems and arthritis. Additionally a veiled chameleon is also pretty picky with the source of its drinking water.
In nature a Yemen chameleon will drink water from plant leaves, for example after rain or in the morning when there is dew on the leaves. The best thing to do in captivity is to mimic this. A chameleon will generally refuse to drink from a bowl of water. Bowls of water are also dangerous for chameleons as they easily drown and it can become a source of bacteria.
You can provide water for your chameleon by misting the leaves in the terrarium with a plant mister. Do this around two times a day. If you don’t have enough time to do this regularly, you can buy a dripper. This is basically a water container with a small dripping valve on it. You can choose how far you open the valve and thus how many drops of water flow out of it in a certain time period. You can allow the drops to fall on the plants in the terrarium. Make sure there will not be any puddle of standing water under the dripper, so let the water be absorbed by a sponge or soil.
Some pet stores sell little waterfalls for chameleons to drink from. These are generally not suitable as they will become a source of bacteria and infections easily. It is hard to keep clean. And again, if the waterfall has a large reservoir of water the chameleon could drown in it.
If your chameleon refuses to drink for a couple of days or needs to take extra vitamins, you can force it to drink with a pipette. Just drop the water into its mouth and most of it will make it in. This is not a long term solution, your chameleon should drink by itself.