Water on movie camera lens?
This is pure speculation with respect to the movie Gravity, but when people use expensive cameras around water, they usually protect them from the water.  It's not very hard to do.  If you search BNH for "Camera Rain Cover," you'll see that you get results for under $10.  To protect the front end of the lens, often people will use a UV filter.  

Also, when you imagine a typical movie camera, do you picture a black box at the front end? Ever wonder what that was all about?  It's called a Matte Box, and the black petals that stick out the front are flags, which keep stray light from hitting the front of the lens and causing unwanted lens flares.  But the other thing matte boxes do is hold sheets of glass in front of the lens for various reasons, most typically ND (neutral density) filters, but also polarizers, UV filters, IR, etc.  Just like the screw-on UV filters, these flat, square ones will protect the lens from water or debris.  As long as the rest of the camera is safe, you're good to go.

To me, this shot looks like the water droplet isn't that close to the front of the lens, so it's probably not hitting a filter that screws directly on the front.  Since there are underwater shots in the film, and since there are shots where the camera swims alongside the actress, my guess is that the camera is housed within an underwater camera enclosure.  These are usually relatively expensive, bespoke pieces of equipment, which can cost as much as the camera they're designed to house.

If you're on an indie budget, it's an item you would rent.  But if Sandra Bullock is your lead, and you've got the VFX budget for a movie like this, your studio probably already owns one, and they'd probably rather just let you use it than A) risk losing their expensive camera or B) spend VFX money on water drops instead of space action sequences. 

Anecdotally, in the movie Children of Men, there's a fantastic long take, where some fake blood hit the front of the lens.  The director chose to leave the blood because he felt it added to the immediacy of the scene.