If someone wants to do deep dives, let them go! Get the training, build up experience and have a go at it. All this talk about 'there is nothing to see' etc, is all subjective and a bit patronizing.
Some people want to do things because they can, or because they like the challenge, or whatever the reason. As long as you know what you are doing (training and experience), plan carefully and accept the risks................
Well said.
I certainly struggle to see how five or six three- and four-metre Silvertips, plus large numbers of Grey Whalers and huge schools of fish at 50-55 metres is 'nothing to see'. Of course, it helps that I'm diving in 86F water with 75-100 foot viz...
And to answer the OPs question:
On air, 230 ft (what can I say, there were three Grey Whaler sharks getting their teeth cleaned on a cleaning station. It's not a depth I'd repeat without very careful consideration of the risks).
On trimix: 350 ft. And yes, it was just for the technical exercise of planning and executing it.