Yes, it's probably cool and relaxing to ride those boats near the Floating Island at Banpo Park in Seoul.
Firstly, the changing colors of the man-made Floating Island make it an enchanting background while your boat is making its way around the calm waters of the Han River.
Secondly, whether you're with your family, your friends, or with your current girlfriend or boyfriend, riding a smaller boat and just floating around the vicinity with plans of having dinner or coffee on the Floating Island are a more convenient alternative to riding the bigger boat of the Han River cruise, whose main attraction at night is mainly the Banpo Fountain Bridge.
I once road at the Han River cruise. The ride is actually interesting if you do it during daytime when you can see the apartments on both sides of the Han River, the bridges over the river, and the Floating Island. Though you won't be able to see the colors of the Banpo Fountain Bridge during the day, at least you'll get your money's worth of seeing Seoul's riverscape under the spring sun.
At night, it's just the lights of apartments and the Banpo Fountain Bridge, which I think has lost most of its colors. It used to be really colorful - with fascinating rainbow colors. But over the years, it seemed to have lost most its colors.
That's why it's probably more fun, especially for kids, to get on these small boats and just laze around the calm, quiet waters of the Han River near the Banpo Park. The kids won't get bored as the ride isn't that long. I think they'd probably even want to stay on the boat longer.
So, one of these cool nights, you know where to boat, float and relax!