Hi. Sounds like an interesting book! So, in 1912, before SCUBA, air was pumped into their suit and the pressure of the air was adjusted within the diving hose and suit itself. Are you familiar with diving pressures, Boyle's Law, Charles' Law, etc.? If no, check those out. Anyway, the dive suit in those days was filled with air. This means that it could be awkward to move. Having those large bulbous parts gives them room to maneuver and bend. You'll notice they can't touch their own face, though, because their arms are stuck in the suit. This can be a problem if they have any problems with equalizing pressure in their ears. By 1912, though, suits like the one you link to had been replaced by much more flexible leather suits due to improved water proofing, though the helmets were still metal. As for how deep they could go--I'm not sure, frankly. Someone else will have to answer that part. But there are a few important considerations: The deeper you go, the greater the pressure, and the longer the diver will need to decompress after the dive. They would have to decompress (at 10 to 20 feet below the surface) for many hours if they went down to 300 feet. Many many many hours. So any time they dived to 300 feet, you'd have to count on like 10 hours or so of people on the surface pumping their air while they decompressed, unless they were at 300 feet for only a few minutes. Google "diving tables" and you can find the information about how long decompression would be. They didn't have super reliable diving tables in 1912--there was more research to be done. The US only adopted dive decompression tables in 1912, so they were brand new. If you mess up your decompression, you get the bends (decompression sickness) which is excruciating and often fatal if untreated. Another issue to contend with is that any deeper than around 110 feet or so and you will experience "nitrogen narcosis"--basically, nitrogen at that pressure or greater makes you high. Sounds amusing, but it's extremely dangerous. Many experienced, badass, tough divers have died because they became so completely high that they pulled their regulator out of their mouth, and other similar stuff. My recommendation is that if you want to make your novel more realistic, I'd keep them diving in relatively shallow waters--50 feet or less. There are too many real life complications and hazards of diving deeper. At the same time, I think a really exciting aspect to your story is that they'll be doing this very dangerous, highly skilled thing. Just looking at those helmet divers is terrifying--it's claustrophobic and you're completely trapped and at the mercy of the people above. If they drop you too fast, you could rupture your eardrums. If they're too slow and you're in the midst of an emergency, you could drown. I'd emphasize the many dangers, and show how they face them. ANYONE getting into diving aparatus was pretty daring in those days. But the thing is, decompression sickness isn't something that yields to bravery or courage or intelligence or heroics. It's simple biochemistry. Oh, and note that dive suits don't come with flippers. Those things are extremely heavy and you are meant to walk on the bottom, slowly and trudgingly, not swim gracefully as with SCUBA. Oh, also: it's dark under the water. Your guys would have to have some kind of waterproof light source, if they went deeper than a little bit. These things start to get expensive. Check out "Timeline of diving technology" on Wikiipedia, as a place to find some sources for you to check out to get more detailed info. I think that the photograph you link to above was a very antiquated model of helmet dive suit. By 1912, divers could wear more flexible leather suits given the improvements with waterproof technology. Good luck, sounds like a really interesting premise!