A leek is a plant and can be grown in a garden. Leek is a homophone of leak. "Leak" is a word that describes a container slowly losing its contents, or it refers to the hole causing it. In this case, the garden hose is the item that has the leak. "I couldn't get to the leek without getting wet" can be understood as the person couldn't get to the plant without getting wet. It can also be understood as "I couldn't get to the leak without getting wet" as in, the person couldn't get to the hole the water was coming out from. It can be accidentally understood this way because leak and leek are homophones. This joke is playing on the word leek.
Our garden hose was damaged yesterday , and I couldn ' t get to the leek without getting wet .