We finished our morning's hike to the northern end of Isla Del Sol, just in time to reach the town near the northern ruins, 15 minutes after the last public boat back south took off. We got there at 2pm, and the boat had left at 1:45pm. Bit of a bummer: especially since various locals had assured us that the boat didn't leave until 2pm. But hey, I guess you can't exactly trust the locals' word around here. Anyway, we had to share the cost of a private boat back instead.
When we got to the northern-end town, there were two private boat drivers sitting in the little bay, waiting to offer us a ride. Initially, one offered us a ride for Bs. 120, and the other offered a ride for Bs. 150. After a big of friendly bargaining, five of us managed to get on the boat for a cost of Bs. 80, meaning that we paid Bs. 16 each (about USD$2). Not too bad — but still, would have been good to catch the regular boat. I reckon it's a scam: the locals are paid by the private boat drivers, to tell you the wrong departure time of the public boat, so you get there late, and have no choice but to find your own transport back.
So, we got the boat ride of about 45 minutes, from the north to the south end of Isla Del Sol. And from there, Chris, Pascale, and myself caught another (public) boat back to Copacabana, on the Bolivian mainland.