ROLLING THE DEEP is the story of the cruise ship Atargatis, sent to find mermaids or fake it well enough to make an entertaining documentary that played fast and loose with the truth. Originally published before its better-known full-length follow-up, INTO THE DROWNING DEEP, ROLLING IN THE DEEP is a bloody horror story of curiosity and corporate greed driving a whole ship of people into a bloodbath at the hands of "mermaids" which evolved to be our predators. 

It doesn't seem like it's specifically trying to leave something for later. Even knowing that a sequel eventually was made, this is a complete story that can be enjoyed on its own. The cast is pretty large for a novella but stays focused on a subset of named characters to make it manageable.

I like it, I just wish I'd been able to get a hold of it before reading INTO THE DROWNING DEEP, but this is harder to find copies of.