What it actually is
Passenger cruise boats run scheduled routes along sections of the Han River, typically departing from piers near Yeouido or Jamsil, with daytime and evening/night options. Trips generally run somewhere between 45 minutes and just over an hour, taking in bridge views, the riverside park skyline, and (on evening cruises) the city's night lighting.
The honest case for it
The night cruise, specifically, gives a genuinely different perspective on the city than anything at ground level — bridges lit up, the skyline from the water, and a slower pace than a typical sightseeing day. It pairs naturally with an evening already built around Yeouido or the Han River parks.
The honest case against it
Daytime cruises are more skippable — the views are pleasant but not dramatically different from what you'd see walking the riverside parks for free, and the time commitment competes with denser sightseeing elsewhere in a short trip. It's also weather-dependent in a way indoor attractions aren't.
Our take
Worth it specifically as an evening activity if you have 4+ days in the city and want a lower-key evening option beyond nightlife districts — skippable on a tight 2–3 day first visit where palace and neighborhood time is more valuable.