Catechism of the Catholic Church §1220 — excerpt
If water springing up from the earth symbolizes life, the water of the sea is a symbol of death and so can represent the mystery of the cross.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.