Peter,
I have been fighting this myself for over two years. I can offer you my own personal story with the caveat that it's my own, and nothing in it should be taken as a recommendation for any particular treatment.
I started associating my tinnitus with dives in the hyperbaric chamber about 2 1/2 years ago. The interesting thing was that it didn't get worse immediately afterwards, it was worse the next morning when I woke up. At first it was intermittent, and it slowly progressed to where it was happening after every dive. I never had any difficulty equalizing and inner ear DCS was ruled out. I was worked up extensively by our ENT and diving medicine physicians here at Duke and nobody could figure out why it was happening. It slowly progressed to the point where I can't even go 3 feet under water in a swimming pool without the symptoms getting significantly worse within 30-60 minutes. I've tried magnesium threonate (some evidence that it helps rats in the acute phase of tinnitus, long shot, didn't help) and I've been taking 240 mg of ginko biloba every morning for roughly 6 weeks (Navy diver lore).
The first time I had an aggravation of the tinnitus a number of years ago, I got a device called Neuromonics from our audiology department. It played a masking noise customized to my own hearing loss along with a musical track. I had a complete remission of symptoms while using it. The second time around it has not helped.
There is also a Neuromonics app that you can download, also adjustable to your own hearing loss, and it's about 1/20 the price of the device. You have to have earbuds with excellent high-frequency range; I use Bang and Olufsen 3i since those are what came with the original Neuromonics device. I've been using the app pretty regularly holding out some hope. Over the past couple of weeks the tinnitus seems to have gotten better, but I slept with a fan on last night and woke up with it much worse (white noise sometimes aggravates it). It tends to wax and wane like that, but this latest waning was my longest period of relative comfort in about a year.
There is some evidence that repetitive transcranial magnetic stimulation (rTMS) is helpful for severe tinnitus. It's off-label since rTMS is only FDA approved for severe depression, but our rTMS folks here have evaluated me and I'm hopeful.
If I had it to do again, I'd have stopped diving earlier. I have no evidence to support this, but I feel like the last couple of chamber dives really did me in. Again, nobody has been able to pin this on diving, but the close temporal association is hard to ignore.
I hope this helps. I would strongly recommend that you be evaluated by an ENT physician who is an otologist and who has experience in evaluating divers, if you haven't done so already.
Best regards,
Eric