DIY under the sea mermaid birthday party ideas. Tissue paper number. Who doesn't love mermaids?! This is genius! So perfect for kids birthday parties! Under the sea and the little mermaid as a party is awesome! So many DIY ideas that are easy and cheap. Which is even better since we done want to break our budgets throwing a mermaid party. I like the food, dessert, decorating, activity ideas! Love it saving it for later!