Why cant devil fruit people swim?

Photo: Toei Animation
Many Devil Fruit powers are whole-body experiences - and that includes body parts that shonen anime doesn't usually talk about, like the genitals.

Thanks to his Gum-Gum Fruit powers, Luffy's downstairs area can stretch. Jozu's junk turns into a diamond whenever he activates his Sparkle-Sparkle Fruit. Buggy can use his Chop-Chop Fruit to detach his genitals and send them flying. If for some reason Marco ever lost his manhood, the Tori Tori no Mi, Model: Phoenix would let him regenerate it. Gladius isn't quite so lucky: his Pop-Pop Fruit can make his thing explode.

This one isn't about genitals, but it's tangentially related so we'll include it. Using her Flower-Flower fruit, Nico Robin can sprout her breasts, just like any other body part.
Photo: Toei Animation
One major drawback that comes with eating a Devil Fruit is that the user stops being able to swim. Some fruits, like the Sand-Sand Fruit, are directly weak against water. A fan wondered how Crocodile - the character who ate the Sand-Sand Fruit - manages to get clean. Oda replied with some in-depth info about how bathing works for Devil Fruit users in general, and for Crocodile specifically.

"First of all, let's discuss the problem of Devil Fruit users bathing themselves. People who have eaten a Devil Fruit are "hated" by the sea, and cannot swim. The "sea" here can refer to anything from rivers, pools and baths to any kind of standing water. On a worldwide level, they are all the "sea." When these people enter the water, not only can they not use their powers, they have trouble moving their bodies at all. They might be able to struggle a bit, but it wouldn't do much good. That's if their entire bodies are submerged in the "sea." With less than half the body or just the limbs, it gets easier. Also, rain and dripping water have no effect at all. Therefore, hip baths or showers are the most common choice. Now, in Crocodile's case, "water" is the weakness of his very powers, so his abilities are robbed even in the shower. But it's not like there are enemies around when you take a shower, so I'd bet he would take them even with his powers being blocked. Don't you think?"
Photo: Toei Animation
According to Usopp, it's not possible for there to be multiples of the same type of Devil Fruit. But as a fan points out, this doesn't exactly make sense. Oda previously stated that Luffy's Devil Fruit - and its effects - had been published in a book, which means that at least one person must have eaten the Gum-Gum Fruit and developed its powers before Luffy did. This isn't a plot hole - Oda has an explanation for it.

"Very sharp of you. But I'm cool. I haven't made any mistakes. As a hint, let me rephrase what Usopp is saying. "The same powers don't exist twice AT THE SAME TIME". How's that? For more detail, you'll just have to wait for a certain professor to make his appearance in the story, and explain exactly what the Devil Fruits REALLY are... Eventually."
Photo: Toei Animation
What happens if two people eat from the same Devil Fruit? Can they both gain powers? The answer is a resounding no!

"The Devil Fruit Abilities are transferred after one bite. After that, it's just a nasty old fruit. In the instance that they take a bite at the same time, the power will belong to the one who swallows first. Nothing will happen if anyone else takes a bite. So in summation, a single fruit cannot give its powers to multiple people."

Submerged Devil Fruit users losing their strength. The most prominent consequence of eating a Devil Fruit is the weakness to seawater: the sea hates and rejects Devil Fruit users, causing them to become weak and helpless while submerged in water, also referred to as "becoming a hammer (カナヅチ, kanazuchi?)".

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