What is a cave on Kauai that is only accessible by boat?

The correct answer is Fern Grotto, which is a popular destination on Kauai known for its stunning natural beauty and unique geological features. Fern Grotto is a natural cave offering visitors breathtaking views of lush ferns and tropical foliage, and it is primarily accessible by boat tours that navigate along the Wailua River. This makes the experience quite special, as visitors can enjoy the serene atmosphere of the river before reaching the grotto.

In contrast, the other options listed are not primarily known for being accessible exclusively by boat. Secret Cave, Waimea Cave, and Pele's Pool may offer unique experiences or sights on Kauai, but they do not have the same level of accessibility via boat or the same reputation as Fern Grotto in this context. This distinction highlights the significance of Fern Grotto as a notable site that fits the criteria of being boat-accessible, contributing to its popularity among tourists exploring the island.