Space Suits Are Scuba Gear

Sci-fi space suits inevitably have exposed tubes leading from the air tank to the helmet, like twin-hose SCUBA gear. Since space suits are fully pressurized, not just the helmet, these serve no purpose but to be yanked out either to provide a moment of tension or an easy way to end a hand-to-hand fight, depending on if the luckless wearer is a hero or a villain. Even if reconnected, this may lead to an oxygen crisis.

Real space suits have no such dangerously exposed air hoses. Real SCUBA gear has a backup regulator should the first one go bad and the hoses are low pressure. Either sci-fi writers are making commentary on a dystopian future with no safety oversight, they've only ever looked at SCUBA gear (before the 60s that would be everyone) and at other fictional spacesuits that exhibit this trope, or they just need a cheap excuse to add tension.

Examples of Space Suits Are Scuba Gear include:

Film

In 2001: A Space Odyssey, the space suits had an attached air line. When HAL's pod attacks Frank Poole it rips out his air line, causing him to die of vacuum exposure/lack of air. Watch it here, 0:50-1:00, as Frank frantically tries to re-attach the line.
Inverted in The Abyss, where Bud's deep diving suit looks a lot like an armored spacesuit.
In The Empire Strikes Back, an infestation of mynocks required the crew of the Millennium Falcon to venture outside in whatever paltry atmosphere can be found on a large asteroid. They wear face-mounted masks with tubes leading back to oxygen tanks.
- Presumably Han issued Chewie and Leia a set of earplugs and corks before they stepped outside as well.
Used in Armageddon so that Bruce Willis can subvert his future son-in-law's Heroic Sacrifice by yanking his air line and taking his place.
Happens in cheapy productions like Missile to The Moon where they just used air force jumpsuits, helmets and oxygen cylinders.

Literature

The cover of Robert Heinlein's Have Space Suit—Will Travel, shown as the page illustration, which was published in 1958. This illustration is also fairly true to the way the suit is described in the book.
- This trope is, however, extremely averted by other suit designs later in the same book. By comparison with Vegan suits, a Latex Space Suit is bulky.
The Moonmaker, sequel to The Man Who Rocked the Earth may be the Trope Maker - some of the original illustrations (see here) were combined shots of the fake Moon landscape and real scuba gear - the closest thing available. After all this was one of first relatively "hard" Sci Fi, made in 1915/1916.
The Clock Punk spacesuits in The Last Hero. Possibly justified since Leonard's notes reveal that they are using repurposed diving helmets.

Live-Action TV

In the Star Trek: Enterprise episode "Damage," a cracked-out T'Pol, jonesing for some literal rock, falls into a depressurized cargo bay, leading to her air hose coming undone and a frantic struggle to put it back.

Video Games

In the old Lucasfilm adventure game Zak McKracken and the Alien Mindbenders the lead character builds a spacesuit out of SCUBA gear (the helmet is a fish bowl sealed with tape). Obviously, not the most serious of games.

Western Animation

Futurama has the classic air tank and hose.
Jackie Chan Adventures does this in the battle with Tso Lan, the Moon Demon. The good guys have the ingredients for the spell to banish the demon again, except one of them is inside Jackie's suit. Tso Lan gloats that Jackie can't remove his helmet without killing himself, to which Jackie replies "Let me air this out" and briefly unhooks his own air hose.

Real Life

NASA's Neutral Buoyancy Laboratory (NBL) at Johnson Space Center. Because space suits are conveniently airtight and pressurized, they actually do work underwater just as they do in space, and because of that, NASA actually trains astronauts by using a 40' deep pool within which the suits will feel weightless and movement will be similar to that in space (albiet with much much more drag).