"Sea Shell" was written by Amy Lowell. In the poem she refers to the"Spanish Main". It's the area of the Caribbean and the Gulf of Mexico that was controlled by Spain.

Sea Shell - American Children's Songs - The USA - Mama Lisa's World: Children's Songs and Rhymes from Around the World  - Intro Image
Sea Shell - American Children's Songs - The USA - Mama Lisa's World: Children's Songs and Rhymes from Around the World  - Comment After Song Image