No, they're not. Moreover, there are different types of Clairvoyance: in the broadest sense of the term, it's simply a skill that affects the acuity of vision.

[And it's fortunate that Watanuki's met Gilgamesh. Now they have more examples to work with!]

Mine, for example, allows me to see everything in the present, and my gift of Prophecy is separate from it. Solomon, the King of Mages, is able to witness both the past and future.

Lastly, we have Gilgamesh, the god-king of Uruk. [His gaze slides over to Watanuki to catch the youth's expression.] Although his is a Noble Phantasm and is continuously active.

[Sha Naqba Imuru: The Omniscient Omnipotent Star.]

I believe he restrains its use for practical reasons, but when employed his Sight allows him to discern even the deepest of secrets.

Undoubtedly, he was in the middle of employing it to discern any possible alternative outcomes to the... situation you foresaw when we arrived.