By sea section.
Explanation: This joke is a pun that plays on the term “C-section,” which refers to a surgical procedure for delivering a baby, and “sea section,” relating it to a mermaid’s oceanic context. The punchline humorously suggests that a mermaid would give birth in a way that combines these two concepts. The humor arises from the clever wordplay, creating a whimsical connection between mermaids and childbirth while eliciting a chuckle through the unexpected twist.