There are a few things that a cinematographer can do to protect the lens from get splattered. The first is to use a lens hood, or other type of baffle that sits just outside the field of view of the lens and keeps not only water/blood/food/special effects, but also lights, the sun, etc. from hitting the lens, too. This protects against lens flares as well as splatter from nearby objects. It can either be something that is directly attached to a lens, or it can be something put up near the camera.
The other technique they can use is to use a longer lens. Shorter lenses have a wider field of view, while longer lenses have a narrower field of view. But they can both get the same objects in a picture if you move the camera with the longer lens farther away. This has the added advantage of putting the lens far away from the splattering item.
For example, here's what a lens with a 45° field of view overlaid with a lens with a 30° field of view looks like when they focus on the same thing. You can see that the lens with the smaller field of view has to be further back to capture the same area: