This is a difficult question to answer because you are asking us to peer into the minds of long-dead, largely-illiterate people. We need to connect dots - rare under the best of circumstance - to determine what people believed, what they practiced, and what they believed about what they practiced. My mentor, Sven Liljeblad (1899-2000), spoke a great deal about this issue and about the attempt of Lutheran ministers to "re-convert" Scandinavia, believing - perhaps accurately - that the initial medieval conversion was at best superficial. Sven actually witnessed ministers in the first decade of the twentieth century interviewing peasants to see if they needed remedial instruction. The following is a paragraph that hints at an answer to your question. It is from Sven's "Introduction to Folklore" (1966), which I am in the process of translating into English of the twentieth-first century (his English was "quaint") and which I am augmenting. But this paragraph is 90% from Sven:

Before industrialization, Europeans regarded certain bodies of water either as entities in themselves or as the abodes of supernatural beings. In some cases it appears that they required sacrifices. The idea that the water being appeared as a horse is widespread throughout Indo-European language areas. But there is a similar tradition in China. In Germanic cultures, this creature is known as the niccor or nicker. Its association with Odin is underscored by the fact that the Norse god rode Slepnir, the magical eight-legged horse. Until the sixteenth century, Scandinavian peasants frequently made sacrifices to Odin at pools in the forest. Similarly, the Greek god Poseidon was associated with water, and he rode a chariot drawn by sea horses. Recently-collected folk traditions from Northern Europe depict the niccor as an old man playing a violin. There are numerous legends describing ambitious musicians who learned from this supernatural being.