Yes, if you're completely deaf in your AN ear, cochlear implants (CIs) are the only option to achieve any hearing in that ear. BAHA and CROS units capture sound on your AN side and transmit it to your good ear, which can be super helpful, but they can't do certain things (for example, they won't help you localize sound, which requires 2 functioning ears.)
But damage to the auditory nerve by your tumor and/or tumor surgery can impact the success of a CI, and unfortunately there's no good way to determine in advance if you will be helped by one. There ARE some tests, but they're not very reliable. I had a CI put in last July (after fighting with my insurance company for 8 months - another story...), and while they were doing the implant surgery they did some testing of my auditory nerve functioning. My physician said the test results were poor, but my CI is working quite well for me. He said this is why he doesn't do that kind of testing in advance - it's unreliable and not predictive of success.
If you and your physician decide to give it a go, realize that you'll face an uphill battle with your insurer. Don't give up, and use all your appeals. I was finally approved when it progressed to an independent external reviewer, but my insurer denied every single one of the internal appeals.