It took them 15 minutes to get us through the rough water, and we were on our way. Padro, a professional guide, showed up shortly thereafter and said he'd come to rescue us. We hired him to guide us to El Castillo and tow us through the rapids. The fort was built to protect Granada from Caribbean Pirates who would come up the Rio San Juan to rape and pillage. The river is really pretty easy to navigate.
Young boys run their fathers' pongas up and down.
You just need to do it a few times. This was my second trip on the river, and just since my first trip it has changed enough to make a guide mandatory. When we got to the rapids at El Castillo you could see the river was at least four feet higher just 100 yards ahead. The rapids were 100 meters of white water boiling over shallow rocks. The whole trick to the rapids at El Castillo is to get upriver enough so as to have room to shoot the gap.