The Infinite Sea (Chaos Chronicles/Jeffrey A. Carver, Vol 3)

John Bandicut and his company of alien friends have journeyed from the vast, enigmatic Shipworld back into the Milky Way galaxy. Arriving on a mysterious ocean planet in the "star-spanner" bubble, they find themselves underwater - and sinking rapidly. Below them lies an undersea city, where a race known as the Neri live in a world lit by artificial light and bioluminescence.
Bandicut and the others are immediately captured by the suspicious Neri. The Neri are a people besieged: from above, by the inimical race they call the landers, whose activities are poisoning the sea...from within, by the failure of their own irreplaceable deep-sea factories...and from below, by the Maw of the Abyss, a terrifyingly destructive entity at the bottom of the sea, now stirring back to life after a long silence.
But why were Bandicut and his company sent here? To intervene in the war with the landers? To try to save the Neri from the Maw of the Abyss, as alien powers once saved the Earth? For Bandicut, the discovery of the true nature of the Maw is only the beginning of an answer. Time is running out. They must solve the deadly riddles that threaten to destroy this world.