I'm not sure what it is on a mechanical level (as in, what it looks like, I assume the name isn't by chance though), but VIvec's water face has the practical effect of him wearing it being the only time you're guaranteed to get the truth out of Vivec.
It comes from the ocean, which is too busy to think, much less lie. Moving water resembles truth by its trembling.
So it's... an attitude, a disposition he takes on at times? Not that it matters what it actually is, as long as it's understood.