No, they didn´t. There were episodes in Star Trek, in which they interfered with a non "space traveling" species in hiding. They dressed up and masked themselves to make studies and research the species. It is forbidden to help with technology and make the species aware of the world outside - if I remember right.

But they could help if unnoticed... I guess.

"No identification of self or mission. No interference with the social development of said planet. No references to space or the fact that there are other worlds or civilizations." that was said in a 1968 episode and they usually talk at length about the importance of the Prime Directive in other series.

There are exceptions, seen in a lot of episodes. I think preventing a civilisation from dieing is ok ; )

Here is a list:

http://memory-alpha.wikia.com/wiki/Prime_Directive

@Je_suis_le_vampire_Lestat said:

When they helped that village at the start with the well, was that not very against the prime directive. I thought they weren't allowed to interfere with primitive worlds??

Also, if i remember correctly. The reason the villages water supply was drying up was because of something the federation had done in nearby space. So they had to correct the error with as little interaction as possible although they did blow it big time by having their star ship pick them up. Don't they have shuttles

It has been suggested that like in real life politics, the Prime Directive is only used when it benefits the Federation or its representatives. It's a handy "tool" to use when one decides to interfere (or not), and still always be right. There are so many exemptions that its interpretation, if ever challenged, will have to be fought in and out of court.

In short, the Federation is the superior power in this universe, and they determine when, how and if the Prime Directive will be applied.

There seems to be no prime directive as of yet, the character of Michael quoted a non-interference regulation at the beginning of the episode and the Captain said something along the lines of "So we let them dehydrate?"

Gary Seven "interfered" with Earth's development in Assignment: Earth. But since his planet doesn't seem to have a non-interferance policy (quite the opposite, as a matter of fact), I don't see a problem with helping aliens with wells.

Especially since, as Burnham mentioned, the problem with the wells was caused by some kind of accident at Federation mining operations on a nearby asteroid. Although that also raises the question, if the Federation takes all the good raw materials from their system, what will those aliens do for asteroid mining when they get out there themselves? If they're going to be concerned about interfering with pre-historic possible life millennia into the future on a so-far-dead world (re: Star Trek The Wrath of Khan, Enterprise episode "Silent Enemy"...) why would they do something as obvious as asteroid mining in a system that already has sentient life?