Need help identifying novel about water planet with mams, moonsteeds, mermaids, and singing

Question

I read an excellent book a while back that I think was written in the late 80s or early 90s.

• It was set on a planet in which almost the entire surface was covered with water.
• There were creatures called "mams"
  • They were like our whales, dolphins, porpoises, etc.
  • They were intelligent and certain people (I think they were some sort of priest class) could sing to them psychically and then ride them to get around the planet
• The protagonist was a young man who had a broken past
  • His mother was some sort of spiritual leader who had had two children - himself and a long-lost sister
  • The sister had been genetically mutated in some way that she became part of a special class of "people" that were like mermaids and always lived in the water. - The job of the mermaids was to save humans from these predatory creatures
  • The creatures lived on the sea floor and called psychically to humans to swim to them, and then wrapped them up and drowned them before dissolving them slowly.
• The main character (the young man) discovered that he was very talented at the psychic singing
  • He imprinted on a massive white porpoise-like creature that became his "moonsteed".

What is the name of this story?

Answer 1

This is Drowntide by Sydney Van Scyoc. I can't add much, except that the boy didn't know he had powers, and his mother sent him to find his sister; as she was the only possible heir. There was some type of sea-grass that helped him unlock his powers. He also had an older sister who died in a storm.

Here's a summary from the 1988 Science Fiction & Fantasy Book Review Annual.

The Drowntide, as the Netholors and Adenyos call it, is the Gathertide to the Sea Tide folk, and the Rermadken; it epitomizes the complementary attitudes towards the ocean held by the land- and sea-dwellers. Keiris, born of a union between a land-mother and sea-father, is the person who explores the sea world and makes what fusion of the two is possible. In so doing, he confronts the fears that come of first entering the water -- of relaxing and giving himself up to its flow; the opening of a telepathic consciousness shared by gifted people and sea-mammals (both having escaped a polluted earth in the forgotten ancient past); and also the vestiges of an aboriginal mermaid-like race.