The cat hisses at the boat, and then abruptly it struggles free, using its weight, the fact that cats are in fact liquid, and also claws if necessary to break free.

And dashes into the boat. Sadie can, if she looks very closely, see a tiny flurry of motion scurry out of her line of sight where the cat is headed. It's too late what it is, but it seems like the cat spotted something interesting to hunt in the boat.