So you are on vacation - why do you need your cellphone? Fact is, cellphone usage at sea is expensive, regardless of what your normal unlimited plan is.
As you board your ship, collect all of the cellphones your family has, and turn them off. If your kid sneaks a few calls while on board, you may be surprised when you get a bill in the mail for hundreds of dollars. Cellphone use should be "emergency only" while on the ship, unless you are prepaired to pay a lot of money for the convenience.
Cellular at Sea. Regardless of whether or not you have an unlimited voice, data, and text plan, you will not be under those conditions when on board. Virtually all of the major cruise lines use Cellular at Sea, also called Wireless Maretime Services is a joint venture of AT&T and MTN Satellite Services. If in doubt your ship is covered by this service, check this Cellular at Sea participating ship list.
Of the majors, only a couple ships in the Celebrity fleet do not use Cellular at Sea. Celebrity's Eclipse and Silhoutte for example use "On-Waves" On-Waves.
When is this service active? Generally when you are in port, the cruise ship turns off the Cellular at Sea service, so that you will connect to land-based towers. If you are in a US port, your normal cellphone plan normally applies. However, if you are in a foreign port, then international roaming charges typically apply.
When the ship leaves port, the Cellular at Sea service is turned on. The ship essentially has it's own "tower" that your cellphone connects to, but then the ship connects to the cellphone network via satellite. If you can't see land, you are probably using Cellular at Sea.
Charges. Using Cellular at Sea is not free. However, the charges you incur depend on your cell phone plan, and you will have to consult your provider for them.
The above rates are sample rates, and may or may not be what your provider charges. Also note that incoming texts have a charge, so just having your cellphone on may result in charges. If you have socially active teenage children, you could easily incur $1,000 per day in cellphone charges - so at least confisate their phones. You are on vacation - why do you need your cellphone?
Alternatives. You may also use the ship's telephone system to make calls directly to and from your stateroom. However, this is generally more expensive than cellphone use, and are typically in the $7.95 per minute range. So if you have to make an important or emergency call, using your cellphone is the cheaper of the two. And if you can wait until you reach a port (even a foreign port), international roaming is usually less expensive than Cellular at Sea.
Some ships support Cellular at Sea data usage, however, it is usually more expensive than other internet offerings on board. Again, check with your provider before leaving for details of what is offered.
Most ships have more or less an internet-cafe, and will also support internet via WiFi around the ship. Some ships offer this in your stateroom, and other ships have internet-enabled TVs as well.
Regardless, rates are usually less using the ship's internet service, typically $0.55 per minute. However, many ships offer internet packages, whereby you purchase a block of time at reduced rates, sometimes as low as $0.20 per minute. However, be aware that internet connections are via satellite, and depending on how many people are using it, can be quite slow. Activities such as email and facebook usually work well enough via the link speeds, but don't plan on uploading many 10Mb photos.
Bringing your own WiFi device on board with email capability and connecting to the internet only to transmit or receive email is a good way to keep in touch with family at a minimum cost.
Another alternative, the one cruise ship members often use, is to use the internet on shore at the various ports-of-call. Some ports have low-cost internet packages, while other ports offer free internet. Ask your cabin steward where the free internet WiFi hotspots are for each port, as they will know.
Cellular at Sea