Yes, the cabin is pressurized.
You're cruising at 33,000 feet above sea level, give or take (10,000 metres). The pressure outside the metal tube containing you is about three-tenths of a standard atmosphere. That's equivalent to standing on the summit of The Mother of the World (Chomolungma, a.k.a. Mount Everest).
The interior of your metal survival suit is pressurized to about three-quarters of a standard atmosphere. That air has to be pumped in under pressure. And heated, of course. Air is cycled as needed, with outside air introduced to keep things tidy. But it costs money (fuel) to heat that new air, so the company gets to draw a benefit/cost line.
Should the craft lose pressure, anyone who's still offgassing from recent dives might not be a happy traveller. That's regardless of cabin pressure goal. Even if the cabin were pressurize to one full atmosphere, are you willing to take the chance that cabin pressure won't somhow fall to the point that you will experience DCS symptoms? A rare occurence, granted. We each get to make the risk vs. reward calculation when deciding whether to squeeze in one last dive on the morning before our evening flight.
The masks will deliver three to five minutes of oxygen max, in the case of sudden decompression. This is all you have but it's enough for the crew to drop safely to around 10,000 feet where the air is thick enough to breathe. I have spent time at 17,000 f.a.s.l. without supplemental oxygen. I was surprised how quickly energetic action became difficult. Hence the pure oxygen, so passengers can understand and respond to crew orders. That's my theory, anyway. I suspect the controlling agency (FAA?) has a different take on this.
I'm not a pilot but I have taught scuba to some, and regularly buddy up on trips with others. I like the way they think, they way they plan dives, their general composure in the face of delays and snafus. I have learned a lot about diving by talking to pilots about flying.
-Bryan
