When we boarded the boat it was totally filled. We were given the requisite life jackets that we had to put on for about 5 minutes.
The trip up the Rio Frio was beautiful but I think we'd have seen more wildlife earlier in the day. The trip took about 2 hours to San Carlos.
On our arrival at the San Carlos dock there was a wait in line to get entry visas and the check-in for the boat to El Castillo was just up the street. We didn't stop to look at anything but went directly up to catch the next boat. We arrived to find there was enough room for us on a boat leaving in 5 minutes for El Castillo.
The boat ride on the Rio San Juan from San Carlos to El Castillo takes about 2 hours and stops a few times along the way for people to get off.
We'd heard from some people on the last boat that the 'newer' hotels in El Castillo were booked up so when we arrived we didn't even try the hotel above the boat landing. Instead we turned left and went around the point. We saw the sign for the Richardson first and went there.
They had a choice of rooms to pick from. Clean with comfortable beds. And cheap!
After spending the night in Los Chiles, Costa Rica we intended to make our way up to El Castillo in Nicaragua. I'd read (online forums and internet) that there was a ferry from Los Chiles to San Carlos around 10-10:30AM. We got down to the dock, got our exit papers stamped, and found that the first ferry wasn't until noon! They were timing it's departure with a bus arriving from San Jose. What I was mostly concerned about was being able to catch a boat in San Carlos. From what I'd read I definately didn't want to stay the night there.