I recommend the New River boat ride to Lamani or the Belize River boat ride to Altun Ha while in Belize. Book with the cruise line or book independently.
We were on the NCL Sun & before we left home we booked with Reggie's Tours. Although my wife was a little hesitant, he was waiting exactly where he said he would be. We only had three people in his van that day (he has 2 busses if needed) & two more people joined us at the river. We saw many others from the ship in large groups. I definitely would recommend his tour & save yourself $45 per person.