I remember loving this book when I was in middle school It took place on the planet Venus. It was all ocean. There were sea horses (their leader was named Belleraphon) and porpoises and mermaids. A group of siblings go to Venus with their uncle(?) and while exploring become involved in a quest to rescue a mermaid from a monster that keeps her locked up in cave (bowl?) with a lid that only he can remove. There was also something about the mermaid's comb and a mountain (reef?) that had chameleon-like abilities.
Encounter Near Venus by Leonard Wibberley (1967)
From the Kirkus review:
Four children (two brothers -- two sisters) are sent to their mysterious Uncle Bill's house in a Colorado wasteland for an eight week summer vacation. Before their plane lands they have managed to spot two flying saucers (one pink). Equally colorful adventures are in store as they discover that Uncle Bill was a passenger on the pink one and is a frequenter of ""Nede,"" a satellite of Venus. It seems that Bill, previously not a fancier of Children, was totally unprepared for their visit, let alone their discovery of the tinkerbelle lights that communicate in Morse code. There's nothing left for him to do but to take them with him on an important mission to Nede where they find Wibberley's version of Fantasia complete with mermaids, baby gorgons, Cockney centaurs, Irish sea elephants, dolphins and a nefarious serpent named Ka, the Smiler.