I work in metric, but essentially it's the same calculation.

10 metres below sea level is double atmospheric pressure. 10 metres is 33 feet roughly.

100 ft below sea level is three lots of 10 metres.

Only thing is you are wrong about the final amount.

at 100ft deep (roughly 30 metres) it's three atmospheres of pressure from water PLUS the pressure of the actual atmosphere.

So it's actually 4x the pressure at 100ft deep.

The difference between space (0 pressure) and our atmosphere(1 atmosphere in pressure,) is 1 complete atmosphere, so its the same as comparing sea level to 10 metres (33 feet) underwater.

Source: I used to scuba dive.

The reason scuba tanks are rounded, is not because the pressure at the depths people dive to are particularly high... its the opposite. The air inside the tank is pressurised way WAY higher than the water pressure. this is to fit over an hour worth of air into a tiny space. If you only needed a few lungfuls of air, it wouldn't matter at 33 feet/10 metres if your tank was rounded or square.

Likewise in space, you don't have to round your living capsule to deal with 1 atmosphere of negative pressure outside you.

It is stronger, so we do it anyway for better safety margins (for example in case of an explosion, or in case the air tanks leak into the capsule.) A cylindrical or spherical object though does use the least mass of materials to make a certain diameter though, which is a bigger reason why most space vehicles are roundish. Every Kg counts when getting into orbit, and within the atmosphere rounded rockets also create less drag and so it tends to also encourage round or cylindrical or conic designs.

But in terms of raw viability, a perfect cube made of metals with people inside it breathing air at one atmosphere of pressure as they would be comfy in should we wish to make one, would be totally able to cope with the vacuum of space, no problems..