You retain the copyright of the code and material that you publish on the Stack Exchange sites. You only license it under the CC-BY-SA license (version 4.0 currently) to everyone in the world. No disclaimer or watermark will change that. If you do not agree with it, you cannot ask and should instead seek answers elsewhere (within your organization, paid consulting, etc.).
The content stored in the Stack Exchange sites is supposed to be a public knowledge library. The knowledge is also partly in the questions. Limiting its spread beyond the CC's license requirements would make the knowledge less accessible.
You could put part of a question on an external site and only link to it. That way this content would not be covered by the license (and add watermarks and disclaimers there), but be aware that questions must be on-topic without considering any linked content, i.e. you would have to describe all relevant linked external content in the question here. In general, the question text that is hosted here would need to be sufficient to answer the question, and the linked content would only be useful for illustration purposes. (Exceptions to that may apply for Stack Exchange sites like Photography that rely on visual content, and they may have special rules, but they may still require that the image content be available on Imgur under the CC license for example. Ask at their meta for their special rules.)
You could also try to generalize your question and remove as much of the secret content as possible. For example, you could create mock-up visual content just for the purpose of asking here.