The iPhone 7 and 7+ is a often experience this bootloop problem due to water damage ... It is important to know that the smartphone is water resistant, and Not water proof, it can be liken to a resistor, it doesn't stop the current it reduces the amount of current that flows through.
The bootloop might be because two or more ec(electronic components) are bridge together from the water damage , or the the nand chip has been damaged.
The best way to prevent that is to not take your phone swimming .