Marine life is most synonymous with marine biology, which by definition is "The study of plant and animal life in the world's oceans, rivers and lakes." The marine life category focuses on both the flora and fauna of freshwater and saltwater ecosystems alike that would be of interest to kids.
Sites about aquariums, whose primary interest is of education than of business.
Coral reefs, homes of many flora and fauna. Fosters many symbiotic relationships and provides protection for prey.
Deadly sea creatures that if provoked can inflict significant harm. Watch out!