Being a 2nd (out of 3) generation oboe player, I've asked (and been asked) this question from time to time. [The most recent one was last year when my son started taking the oboe.]
The best answer I get is that about half of the oboist swear by soaking in water, and the other half swear by soaking in saliva. The one thing they do agree upon is that there is a difference in how the reed plays if you soak it in different liquids. [My father used to swear by soaking his read in Vokda or Kirschwasser when he had a cold.]
Here are the facts that both sides acknowledge:
- Saliva & Water have different densities.
- The enzymes in saliva interact with the wood of the reed.
- Gunk in the reed is not good
Proponents of saliva soaking will often use arguments such as
- The increased density of the saliva changes the way the reed responds
- Soaking in water and then switching to your mouth means the reed will change as you are playing it as the water is displaced by the saliva in your mouth.
The Opposing view (those who prefer water) argue
- Saliva is gross
- Reeds wear out faster when using saliva
- You can't get all sides of each read using saliva [There is an opening between the reeds when you blow through it, so I don't see this one.]
Where is the truth? [Warning, entering The Opinion Zone]
IMHO, all of these are true. What it really comes down is what the player is used to, how they were trained, and (probably more importantly) how the reed is customized. (Every serious double reed player ends up adjusting their reeds, even if they buy high-end pre-made ones.)
Concerning the "gunk": one thing my father did stumble upon in his 60+ years of playing the oboe was the importance of cleanliness to the survival of reeds.
First off, make sure you mouth is clean of any food before playing. [Brush your teeth just before playing! I actually carry a toothbrush in my oboe case.]
Second, make sure you clean your reeds. I like to clean mine under a mild stream of water from a faucet with tip pointing down so as to prevent damage and allowing water to enter the back.
My father uses an ultrasonic cleaner. He ends up getting several times (~5x) more life out of the reeds he cleans with the ultrasonic cleaner vs. the ones he didn't. I haven't found that much of a difference, but I never did play as much as him. [There are also those who claim the frequency of the ultrasonics is important to how well it works.]