ELI5, if you get an injury underwater (say in the ocean) that opens the skin and bleeds, what stops your body from intaking water?
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Your blood pressure is higher than the water pressure.
That’s basically the short version.
When a hole between two liquid containers is made, effectively connecting them, the liquid flowing through that hole will be from the higher pressure container to the lower pressure container. In this case your body is the higher pressure container and the ocean is the lower pressure container.
Incidentally this effect is also behind why when the city water supply has been shut off for a while, then turned back on, the water always comes out of your faucet a bit discolored (yellow or brown) until you let it run for a while. And you’re usually told not to drink it until you’ve left it on for a while to flush water through the system.
There are always leaks in a city’s main water pipes at the joins where they turn corners or split in two. It can’t be helped. But most of the time you don’t need to worry at all about the leaks because the pressure guarantees the leak goes OUT OF the pipe, not INTO the pipe. But, if the water pressure is turned off for a while, then that stops being true. The goopy mess of dirty water that was created when the water pushed OUT of the underground pipe into the surrounding dirt now flows backward through the leak back into the pipe once the inside of the pipe is no longer under pressure. Thus when the water is turned on again that gunky stuff that leaked into the pipe has to be expelled back out of the pipe again. After that, the pressure will ensure again that the leak goes in the correct direction – out of the pipe not into the pipe.
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