Imu created Devil Fruits that prevented people from swimming, so that he could drown powerful people

The setup

I always found the weakness that comes with Devil fruits slightly bizarre. I could never mentally connect why such an awesome power as the Devil Fruit comes with such a drawback. Especially after Vegapunk revealed that DFs are essentially created by people's dreams, this incongruity became more puzzling to me. It finally clicked into place when it was revealed that Imu's power is called "The Devil's Fruit". In my mind, this implies that Imu somehow allowed for the creation of the Devil Fruits by people. After all, in Oda's box, his power was not presented as the classic, Power Power no mi name, but simply The Devil's Fruit

Chapter referenceChapter 1179Imu's power revealed

The "how" is not important for this theory. I am focusing more on the "why".

A deal with the Devil

It is widely known that the actual Devil in folklore and other stories granted wishes, but never without a drawback. There is always fine print in his contracts, and this fine print usually signifies something catastrophic for the people that accept his deals. Imu follows this pattern as well. He can grant his subordinates the power and strength they desire, but they are forced to obey him through his Covenant. For his enemies, he can grant the same power by taking advantage of their greed and lust for power through Domi Reversi, but the drawback is that they completely lose control of themselves and their free will. So, if my theory that Imu allowed the creation of DF is correct, then why would he grant people this amazing power, and what is the fine print?

Chapter referenceChapter 1181Imu explains how Covenant and Domi Reversi work

An ancient deal

In the Harley, it is stated:

"Within the emptiness there was breath.
The forest god tamed demons and the sun spread the fires of war.
Those of the half-moon dreamed.
Those of the moon dreamed.
Man killed the sun and became god, and the sea god stormed.
And they will never meet."

Chapter referenceChapter 1138The Harley

In my interpretation, this reveals how devil fruits were created so people can become more powerful during the war between the 20 kingdoms and the Ancient Kingdom. Those of the half-moon and the moon dreamed so they can create power through devil fruits. They became very powerful but eventually the sea god stormed.

It is a very prominent theory that Imu represents the Sea God/Devil so it is very possible that this line refers to when Imu caused the Great Flood 800 years ago. So here is how it all connects.

What if Imu created Devil Fruits that would make people unable to swim, so he could drown the most powerful people in the world during his Great Flood?

That is in line with how Imu has been acting so far. Giving people what they think they want in order to control them, but always able to take it back when he wants. He could have given this power to the Kings of the 20 Kingdom Alliance so he could drown them and their allies after he achieved his goal, which was to defeat the Ancient Kingdom. The "betrayal" of Lily could also play an important role as to why he decided to cause the flood at that time. Maybe Lily realised what he was planning, so he caused the flood in an attempt to kill her and stop her from revealing the truth.

A counter argument to this theory is that there are numerous Devil Fruits that can fly so these people would not necessarily drown during the flood. However, this could be explained that these people would have to keep flying for days in order to survive and reach land, or that flying devil fruits were created much later. We would need more information to understand what his plan was exactly.

TLDR: Imu implemented the drowning drawback into Devil Fruits, in order to be able to drown the most powerful people with a Great Flood.