How likely is it that a shark will eat someone?

Considering you're asking how likely it is that a shark will eat a WHOLE person, then not likely.

When shark attacks do happen, the majority of people don't even lose a limb.

They are just left with awful gashes or cuts.

When an attack happens, there are usually other people on the scene,

or the shark gets scared and swims away.

The likelihood of a person being totally consumed by a shark is very slim.

I'd say you have a better chance of inventing time travel by sticking a wet fork in a toaster oven.