English borrows the ocean whenever it needs a word for courage or risk. People take the plunge on a new job, test the waters before a big decision and admit they are out of their depth in a meeting. None of them are wet. The sea is just a handy picture for anything deep, uncertain or hard to back out of.
Scuba divers hear these phrases differently, because for them some of the words are not pictures at all. Diving has its own vocabulary, and much of it sounds figurative while meaning something exact. The first of those phrases a new diver meets does not even happen underwater.
The First Literal Phrase Is on Paper
Before anyone learns to breathe from a regulator, they meet the phrase fit to dive. Every student fills out a medical questionnaire about their lungs, heart, ears, sinuses and medications. A yes answer usually means a physician has to review the diver's history and sign a form before training can start. In that context, being cleared is a literal medical decision, not a figure of speech.
Some divers get the signature from their own doctor. Others arrange medical clearance for scuba diving online, where a physician reviews their records and meets them on a short video visit, since plenty of primary care physicians rarely see the form and some decline to sign it. Either way, the paperwork comes before the water, which is a useful reminder that the ocean idioms everyone uses started with real risks.
Ocean Idioms Everyone Uses
Take the plunge. To commit to something daunting, such as a move, a marriage or a career change. The image is the moment of jumping in after standing at the edge for too long.
Dive in. To start something with energy and without hesitating. Teachers say it about homework and managers say it about projects.
Test the waters. To try something cautiously before committing to it, the way a swimmer puts a toe in first.
In deep water. To be in serious trouble. Someone who has overspent, overpromised or overstayed their welcome is in deep water.
Out of your depth. To be in a situation that is beyond your knowledge or ability. Literally, it describes water too deep to stand in.
Sink or swim. To succeed or fail with no help from anyone else. New employees are sometimes left to sink or swim.
Come up for air. To take a break after a long stretch of intense work.
Make a splash. To attract a lot of attention, usually with a debut or a bold move.
A sea change. A profound transformation. The phrase comes from Shakespeare's The Tempest, where a drowned man is said to suffer "a sea-change into something rich and strange."
Diving Words That Only Sound Figurative
The bends. This sounds like slang for a bad day, but it is the common name for decompression sickness, which happens when nitrogen absorbed at depth forms bubbles in the body during a fast ascent. The nickname dates to nineteenth-century workers who surfaced from pressurized bridge foundations bent over in pain.
Equalize. Outside diving, this means making things fair. Underwater, it means balancing the pressure in your ears and sinuses as you descend, usually by gently pinching your nose and breathing against it.
Squeeze. Divers do not mean a hug. A squeeze is the pain or injury caused when air spaces in the ears, sinuses or mask are not equalized against the surrounding water pressure.
Rapture of the deep. It sounds poetic, and Jacques Cousteau helped make it famous, but it describes nitrogen narcosis, a drunk-like state that can impair judgment on deeper dives. Divers often shorten it to being narked.
Surface interval. Not a figure of speech for downtime, but the measured time a diver spends at the surface between dives so the body can release nitrogen.
Buddy check. A literal routine. Before every dive, partners check each other's air, weights, releases and equipment.
Why Divers Hear Them Differently
Idioms work because the risk in them is safely imaginary. Nobody who takes the plunge on a new apartment is in danger of drowning. For divers, the same words point at real physics, which is why diving training spends so much time on vocabulary that the rest of the language treats as metaphor.
That is also why the literal phrases come first. A diver who knows what equalize, surface interval and the bends actually mean, and who has been honestly cleared as fit to dive, is the one who gets to enjoy the figurative versions afterward. For everyone else, test the waters remains excellent advice.