This activity consists of a 45 minute kayak paddle up the Wailua RIver, a one hour walk through forest, and a brief visit to a really pretty waterfall. For the activity to be worth you time, all three parts have to work out just right. Otherwise, with so many other attractions in Kauai, this isn't worth your time and money. When we went, the kayaking was OK (I doubt it's ever better than OK), the trail was horrendous (unbelievably muddy), and the waterfall, while pretty, had so much water that there was no swimming in the pool around it. If there has been any significant rain in the last 4-6 days, DO NOT GO. The tour operators will not tell you about this until it's too late. If you're not sure, DO NOT GO.
If you do go, take insect repellant. Despite the misinformation from our guide, you will need it.
We didn't like Duke's, but there were other tour operators doing the same tour at the same time, and their customers were as muddy and miserable as us. It's not the tour operator, it's the tour itself. Find something else to do.