Saliva has components that serve a digestive purpose (and help break up food stuck between teeth), but mostly it appears to be just water that lubricates the mouth and helps you swallow food.
So do cetaceans - whales, dolphins, etc. - actually salivate? Or has seawater supplanted the need for saliva, resulting in the glands evolving over time into vestigial form?
According to Whales of the World, whales do not salivate. In some species, the salivary glands are not present; and in others the salivary glands are present but vestigal.
Saliva in the mouth would not serve a digestive purpose for cetaceans, because they swallow their food whole. Even the ones with teeth don’t chew. Dolphins, for example, swallow fish whole. And being underwater, any saliva would just wash away every time a cetacean opens its mouth.
(I could imagine there might be mucus in the throat to help lubricate, to make swallowing easier. That’s just a WAG speculation on my part.)
Here’s something I found.
According to Wikipedia, “Toothed whales have lost their sense of smell, as well as their saliva glands.”
But here are some excerpts from research articles from Mystic Aquarium, including one that discusses the saliva of beluga whales – they are investigating the idea of testing saliva to determine blood levels of various hormones (cortisol is mentioned here) instead of having to take actual blood samples.
Googling dolphin saliva turns up several other similar-looking references (not all of them specifically about Belugas, I think).