Goofy in Giant Trouble

Goofy in Giant Trouble is a book in the Big Little Books series.

Summary
Mickey is a former agent for the World Police Organization recruited to investigate reports of piracy by the infamous Black Pete and his scurvy crew of cutthroats. Goofy slips and falls out of their small plane! Mickey's quick thinking saves his pal by diving the transport and catching the big lug mid-air, but he is swept off the wing and falls into the shallow lagoon, where his foot gets jammed into a coral-encrusted rock.

Once more Mickey rescues him, and the pair makes it to the shore only to discover something quite bizarre. The rock isn't a rock at all.