[anamax]

> a glass enclosed under sea walkway 30m down is not that much harder than building one 20m down

It's 50% harder.

What pressure are the passengers exposed to?

I ask because 1 hour @ 4 atmospheres of 80% N2/20% O2 is risking decompression illness. (Yes, 4 - you've got 1 atmosphere at sea level.) You can reduce that risk by increasing the O2 percentage but if you do that too much, you're risking O2 toxicity.