Human's are not designed to breathe underwater, so technically, no I can't, and neither can you.BUT, we can breathe underwater by using artificial breathing supplies, such as an air tank or snorkel.
Well, you can't really breathe underwater. You can hold you breathe underwater but most people can't do that for more than a minute. If you really want to breathe underwater, try scuba diving on...
Unless you have gills, you are going to need access to oxygen. There is the traditional snorkeling mask, a scuba tank, or more recently, a new breathing apparatus that doesn't use tanks or tubes.
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Fish, and other sea creatures. (Apart from things such as dolphins & whales who can just hold their breath for a long time) Human's can artificially breathe, using air tanks or snorkels.