Here's another vote for TripAdvisor and the Ports of Call section, plus sometimes just googling for private tours in X location. I've set up small independent tours for 20+ years for ourselves and a few others (all met on Roll Calls). We've never had a bad experience with those I've set up, and we've had some truly amazing ones. I've even set up small private tours in places such as Sumatra, Togo, and the Gambia. It just takes research and patience to find the right guide. I respect those who take ship excursions, but we've been DIY people all our lives. I always go to the farthest place we want to see first, and work our way back to port in plenty of time, and I make sure there is plenty of elbow room in the vehicle.