I agree with imsodin that we shouldn't merge the tags. Since this is too long for a comment, I'll add it as a supplement to his answer, which I upvoted, and hope it's not too redundant.
Generic, or umbrella, tags are called meta-tags by the SE system, and are discouraged in many cases. The What are tags, and how should I use them? page in the Help Center has a good description and offers guidance.
Do not use meta-tags in questions. Here are some tips to help you determine whether a tag is a meta-tag:
If the tag can’t work as the only tag on a question, it’s probably a meta-tag. Every tag you use should be able to work, more or less, as the only tag on a question. Meta-tags, like [beginner], [subjective], and [best-practices], are not helpful by themselves – they do not communicate anything about the content of the question.
If the tag commonly means different things to different people, it’s probably a meta-tag. For example, the meaning of the tag [subjective] is, itself, subjective; the same is true for tags like [best-practices] and [beginner]. Best practices to whom? Beginner by what criteria? Use only tags that have a broadly accepted, objective definition.
As you can see, this was written originally for a programming audience. However, we can apply it to this discussion, and our site in general, exactly as imsodin did. For instance, water would be a meta-tag. That means, at least in most cases, it wouldn't be helpful as the only tag on the question. It would also need something specific, such as drinking-water or even water-purification, which was not in gerrit's question here, but I think might be a good inclusion to this discussion.
There's a similar situation at Gardening and Landscaping. One example of a meta-type tag is plant-care.
Part of the definition says:
Use this tag for general questions on plant care. This is an extremely general tag and should be avoided if possible. In most cases there are much more appropriate, specific tags available. E.g. for watering, light, pollination.
The same type of language could be part of our water tag, and emphasized in the tag wiki, which we haven't written yet.
As editors, when we come across a question with just the water tag, we could add the more specific tag, and either leave water there, or remove it, depending on the situation. Sometimes leaving it would be helpful in terms of finding relevant questions, but I concur that we should try to avoid having it the only tag, and that we shouldn't have other tags routed to it.