Xiang Shan and his companions followed the sound of dolphins' cries to find their target. It was a shallow sea reef. Two dolphins were tied up. A rope was tied to their tails, and then the rope was fixed to the reef, with just enough length for them to surface and breathe a mouthful of air.
"Knots," such things, are indeed difficult to handle with just a mouth—whether it's tying or untying a knot, it's hard enough to do with one hand, let alone with a dolphin's snout, which is less flexible than two human fingers.
The two dolphins were also tied far apart and could not help each other untie the ropes.
The two dolphins were very frightened. The oceans of Earth move with the Moon's gravity. It's like a gentle hill following the moon across the sea surface. The water will rise.
The rope behind them won't magically extend.
They also have to beware of being discovered by the nearby patrolling Protectors.
The poor dolphins cried out in fear.
