I have done several of my own shore excursions. I research the companies on line first by doing a search on the ports at Google then I try to find feed back on cruise chat websites such as this one for feedback. It would help if you listed your ports so people can make recomendations.
I have booked with Sunny Liston tours in St. Thomas a real bargain at $20, I wish I had done San Juan on my own as Carnival city tour was not worth it, I would have had more time at the places that interested me if I just took a cab, in Nassau I went to the zoo, took a city bus for just $1 the locals were protective of us, and I felt very safe, I also ate at the Fish fry area where the locals eat. I have also done my own excursions in Mazatlan Mexico with Mazatlan Frank and in Belize with Coral Breeze limited. I plan to use Captain Marvin on my next cruise in Grand Cayman.
I saved a lot of money and I feel had better excursions then the ship offered, with smaller groups. In certain ports where safety might be an issue I might stick to the ships tours, ditto with tender ports as you don't want to miss your boat. Hope this helps.
Carnival Mardi Gras 1987 (Bahamas)
Carnival Legend 2005 (Exotic Western Caribbean)
Costa Mediterrenia 2005 (Eastern Caribbean)
Carnival Pride October 2006 (Mexican Rivera)
Carnival Legend 2007 (Western Caribbean)
Carnival Glory 2009 Western Caribbean
Carnival Liberty 2011 Eastern Caribbean
Norwegian Spirit 2011 Western Caribbean
Carnival Miracle 2012 Southern Caribbean