Well I’m glad you asked! Because “Maris” doesn’t mean “Mary.” It means “sea” or “ocean.” So the title literally means, if you keep the same word order: “Hail, Sea Star.” So where did that wording come from?
To begin with, someone in the 9th century wrote the chant version that is used in Roman Catholic liturgy. There ended up being four different tunes authorized for use in various services. The words present Mary as a merciful and loving mother, with “Star of the Sea” being a title that especially appealed to travelers praying for a safe journey.