Hey Anna. Welcome to cruise-chat. It is easy to book excursions at the port. I can't speak for every one on this topic, but for me personally I book through the ship. Here is why and some suggestions...
1. It is the "peace of mind" aspect that I like about it. The transportation to and from the excursion is taken care of. IF the excursion is running late your ship will wait on you; whereas, if you book thru another online excursion company or at port you are on your own. If it is a popular excursion they usually sell out quickly.
2. If you decide to book on your own just make sure it is with a reputable company. MAKE sure you give yourself enough time to do the excursion and to get back to the ship on time.
3. Check out each city you will be visiting and see what kind of tours and tour operators they have.
4. Also check out shoretrips.com. I haven't used them, but quite a few people have and have said good things about them.
Phew...I think I have rambled on enough...Let me know if you have any other questions. We will be glad to help.
Fascination-September 2007 Key West and Cozumel
Spirit-August 2010- Alaska