Okay I pulled out my manga here, and it makes sense to me. This seems like it's scheduled as part of the trip in some way; Kaorin mentions that her group is headed towards Miyako Island, which is also pretty distant from Okinawa Island. At least in the manga, it seems like it's a whole-day side trip. There's an inland ferry terminal near the falls, and one panel of the manga shows Sakaki and Maya on a boat right before the panel with the waterfall. Also I think it's more likely they would have flown to Ishigaki rather than Hateruma. Possibly even having a night in a hotel at Ishigaki. This all seems reasonable for some kids to do in a day.
"Kitto daijoubu da yo." - Sakura Kinomoto