The D3100 isn't a weather-sealed body, but it probably helps to describe what that even means.
Every camera body is designed so that the body seams "fit" together nicely. In making a nice fit, the gaps through which water can enter are minimal. Although "capillary action" will tend to help water get through gaps if they exist (and they do).
If a camera body is "weather sealed" it means that the manufacturer added gaskets to most (but usually not all) body seams, probably used gaskets on buttons, and probably use o-rings on dials. These are meant to be tight enough to prevent water from leaking through as long as the water is not under pressure. They will probably still leak with water under pressure (never submerge a weather-sealed camera ... they're not actually "water proof" and if submerging is needed, get a dive housing. Those housings actually are "water proof").
Another thing to keep in mind is that even if you have a weather-sealed body, the lens may not be weather sealed -- so there's another component that could be damaged and the lens mounting flange is usually an opportunity for water to leak into the camera body (weather-sealed lenses usually have a gasket/skirt to seal the lens mounting flange.)
There are some low-cost solutions. If your only concern is "spray" then you can pick up plastic baggies designed to fit a DSLR (Keith linked an example of one brand above). The baggie has an opening for the lens (usually an elastic band keeps the baggie snug) and the baggie is usually open on the bottom to allow you to put your hands inside for camera control. In other words, if you got a pretty good splash, it would keep the water out. But if you dropped the camera in the water, it'd still be flooded.
Since the baggies leave the front of the lens exposed and you may not have a weather-sealed lens, this is ONE TIME in which threading on a clear filter would probably be a good idea. Salt water will leave a residue that will crystalize as it drives. You don't want that working it's way into your lens. But if it crystalizes on a filter... that filter can be removed from the camera and washed clean.