Here Be Monsters! An Adventure Involving Magic, Trolls, and Other Creatures; The Ratbridge Chronicles Volume 1 Chapter 9

The Wet Dry Dock

• They arrive at an old unused dry dock that's been flooded, and as they look into the water, they see something moving in it.
• The boxtrolls toss hunks of grass into the water, and the creature inside surfaces: it's a freshwater sea-cow. It looks like a dairy cow that's adapted to aquatic living.
• Fish holds a conversation with the sea-cow, and then he talks to Willbury (who translates for Arthur).
• The sea-cow is upset because her three calves had disappeared from this area, plus the tunnel that leads back to open water has been blocked.
• Nobody knows of any more holes, so they return to Willbury's home. All this stuff is bad news.