There are several ways you can breathe underwater.
The first and easiest way is to simply put a torch at face level, it will replace the water with air, but it be destroyed and pop off the wall quickly.
Another way is to carry some sugar canes (AKA Reeds/Papyrus) and quickly throw one down on your feet, and another on top of it, because of the weird way that water works, you will be able to place them if you are quick enough, but like the torch they will also be destroyed. This way might be better if you are gravel mining underwater.
But, gravel does spawn elsewhere than underwater.
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You mine gravel, sometimes flint is dropped when you break a block of gravel.
Yes it is actually the fastest way to get flint from gravel. There is a small chance to actually get flint from gravel ether way.
Flint is sometimes dropped when you mine gravel.
You can't, unless you're using mods.
Other than using gravel for building, gravel is useful to farm for flint. When you mine gravel with a shovel, there is a small chance that instead of dropping a gravel block, it will drop flint. Flint can be used in crafting recipes such as flint and steel as well as arrows