For a context on this beautiful shrine: Yemanjá or Iemanjá and many other names in different places around the world, is an Orixa in the Candomlé religion which is another big religion here in Bahia. This religion was brought to the Americas from a Yoruba tradition in Africa via the intense slave trade here in Brasil. Candomble was practiced underground for many years, as Catholicism was the mainstream religion, and it was a form of cultural resistance.
There are many orixás, which are something like saints, and Yemanjá who as you can see takes the form of a mermaid, is the ocean, and a symbol of motherhood protector of children. Many sailors also look to her for good luck at sea.
There were a lot of fishers around this shrine, and inside there was this altar and lots of little flowers and candles a lot like altars for Catholic saints. From my experience here in Bahia, where there is the highest concentration of blacks because of slave patterns and the sugar engenhos(industrial style mills), but it seems like a lot of people, like here in my family will have influences of both religions; they seem to have been made quite compatible just like a lot of indigenous religions and catholicism mesh a little in Mexico.
This mermaid form of Yemanja is a very popular representation of her but she isn't always depicted as a mermaid.