This got me curious, & using Expansion anchors in construction applications got me looking. While I couldn't find anything specific to climbing applications & submersion, I did find a reference to 304 or 316 stainless steel being used in submerged marine environments (harsher than typical freshwater) by boat builders.
To my knowledge, no expansion anchors are available in 304 SS, so you would be limited to 316.
I did come across this fastener selector tool which does specifically mention expansion anchors being used in submerged applications (in concrete).
It is worth noting that even bolts installed in high, dry desert locations will need to be replaced sooner or later, so it is a good bet that the more extreme environment will shorten their lifespan no matter what you do.
Also, maybe of more importance here, is making certain not to allow different alloys to come into contact with one another for extended periods. Do not use two different alloys for the bolt & the nut, & if you leave your hangers on the bolts when not in use, make sure that they are stainless steel also. Allowing dissimilar metals to remain in contact with water will result in Galvanic Corrosion. Which essentially means the rusting process will happen much faster.
Just a thought, but you may want to be especially wary if the bolts are installed in sandstone with a high Iron content, as it seems to me that the Iron in the sandstone could react much more strongly with the stainless in a submerged environment. Anyone else have any observations on the effects (or non-effects) of Iron containing sandstone on anchor bolts?
My two cents; give it a try. Maybe just very carefully inspect each bolt before use after a submersion.
Edit: It may also be worth firing an email off to a manufacturer: http://www.fixehardware.com/shop/contact-us
Edit: Edit: Just did, waiting for response.
Edit(x3): Didn't hear back from Fixe, so I emailed Hilti customer service with the following question:
Hello, I am a climber & use your KB3 & TZ expansion anchors to secure my safety lines into rock. A route I am considering setting is partially submerged in melt water seasonally. Would it still be safe to use a SS KB3 or TZ that has been submerged for several weeks? (assuming of course correct embedment & compressive strength of the rock).
This Was their reply:
After speaking with our engineers, the water part would be no issue. However their question is will there be any freezing of the water. Could you possibly call in to Customer Service next week between 8:00am and 4:00pm CST to speak with an engineer on this further? 1-800-879-8000. They had a few other questions about the application before giving a difinitive yes or no.
Inside Sales Partner | Customer Service
Hilti North America
So it sounds as though while submersion is okay, a volume of water collecting behind the bolt, freezing & pushing the bolt out or cracking the rock is their main concern.