In the 1970s, a toy called the aqua bell was available. It consisted of a thin, transparent plastic helmet weighted down with a collar section full of sand. It was supplied air from the surface via an electric bicycle pump. It was rated for 35 feet although one shouldn't really go deeper than 25 feet, as that's the limit before which your body will not saturate with nitrogen. Stay above that limit and the bends will not occur.
Unfortunately it's no longer available, and the only diving helmets you CAN buy start around $4,000. There's no good reason for this. They work the exact same way. Air is supplied from a surface compressor, and because it can freely escape out the bottom, the pressure inside self-regulates to ambient at all times. That's why it's not a struggle to breathe. It's the same way historical diving helmets worked before the advent of scuba, basically just a diving bell you wear over your head.
Why helmet diving? Because it's fun. Your hair and face stay dry. If you stick a radio inside and you're in shallow water, you can talk to the person watching over the pump (although a wired communication system would give better results). You can breathe normally, instead of sucking on a regulator. It's a bit like being in a space helmet, walking on the moon.
Today, helmet diving is still done. Unfortunately it's marketed as a tourist attraction and as a novelty for the wealthy and it's priced accordingly. Even though it's still just a weighted container fed air from a surface compressor, a consumer diving helmet like this one costs $4,000:
Here's a tourist attraction featuring similar helmets:
I consider it ridiculous that this experience is available only as a short duration tourist attraction or at an exorbitant, ridiculous price. There's no need. The same thing used to be sold as a toy for no more than $50! Surely something like this can be produced that everyone can afford so that ordinary people can experience helmet diving whenever they like.
Having done the math, my battery pack has around 336 watt hours of capacity and the compressor is 150 watts. That's 2.2~ hours of run time under ideal conditions. With AC conversion loss and the slight sulfation of the battery taken into account I can be guaranteed at least an hour of air. I'll know when it has run out because the bubbles will stop, and because I intend to have a stopwatch mounted inside the helmet where I can see it so I know how much time I have left. When I'm running low I can simply swim to the surface.
Please, no comments expressing concern for my health. Trust that I have a thorough understanding of the risks involved. I am certain this can be built, that it is safe, and that I will not be injured provided I stay in shallow water and have someone with me for safety. I would not put the hambros through anything I wouldn't also subject myself to, and if everything goes as planned, I will.
Here's the initial design for the diving helmet and a possible partial habitat/observatory I might also attempt following the completion of the helmet.
The helmet can be used in this configuration to get to and from the habitat, and also to explore the area around it. This configuration accommodates one person, so the supply of air is always sufficient, although because the rate of flow is enough to sustain 20 people by my math you could also bring a second person down to the observatory provided another such helmet. The real trick will be finding someplace with clear water deep enough to deploy this thing and getting it there.
If all goes well I intend to make a Kickstarter.com webpage pitching this idea as a commercial product. The goal will be a complete helmet diving system for under $100.