88 % of private insurers include some kind of coverage for cochlear implants. Medicare covers CIs, Medicaid coverage differs on a state-by-state basis -- all states cover them if you are under 21, but not all cover adults.
If you are looking for an individual plan that provides CI coverage and you don't have insurance right now, you are going to be in a world of hurt. Even if the plan normally covers CIs, your ability to get that coverage will likely be extremely (and permanetly) limited based on a pre-existing condition, and that's assuming they will insure you at all.
Group plans (like through work) cannot deny you coverage based on a pre-existing condition. They can, however, put in an exclusionary time period that they won't cover pre-existing conditions if you were previously uninsured, I think the limit is 18 months. So it is a temporary limit, but you will get coverage eventually.
What, who, and how much will be covered is extremely variable and totally based on your plan and individual circumstances. Some of the things it depends on include whether your facility and surgeon are in our outside of the insurers network, and what types of co-insurance and deductibles you have to pay, and what your coverage is for durable medical equipment.
Hope this helps, best of luck, let me know if you have any other questions