It really also depends on where you are going and the specific phone plan you have. For example, my phone plan includes US, Mexico and Canada. So when I went to Cozumel I didn't need a phone package.
Okay so to break down what you'll need/options (coming from a previous AT&T service rep:
[Yes different service but they offer VERY similar things]
1. International plan: Depending on your ports and your current plan you'll want a international plan. Which plan really depends on how much you plan to talk/text. Data is usually unreasonable so I'd just plan not to use data. This plan is used only for when at port. Take a look at the pay per use rates as well, that's what I use but I only use my phone for emergencies.
2. Cruise Ship Plan: To use your phone on the ship itself when you're at sea you'll need a cruise ship package or you'll be subject to the pay per use fees which is VERY EXPENSIVE. The plan is also pretty expensive too so you'll really have to determine if its worth it or not. Many people get the international plan (above) and choose not to use their phone at sea. Keep in mind tho that this means taking time away from valuable port time potentially missing out on things to update family back home.
3. Wifi Package: This is usually the option I go for on my cruises. I purchase a wifi package. Its slow but it works for quick updates and my phone send messages to other iphones via wifi or I can message people through facebook. This was the most economical option for us as we didn't want to spend time at port relaying messages back and forth
It really comes down to how much do you plan to stay connected. For my family (well minus the kids, the first day they all act like they're going to die without their phones/video games LOL), we like to be a little disconnected from the world. It's nice to put our phones down and enjoy our time together and experience these once in a lifetime moments. The most we have needed is the wifi. In a true emergency I could use the text/calling from my phone. In reality if I'm going to send a text or two when I'm somewhere literally that'll all it'll be 1-2 texts which I'd rather do the pay per use for that few. Also make sure you look into your phone plan's actual terms. Between that and determining how much you really need/want your phone will help determine which plan you should get.