Taking the ship is usually faster and cheaper than going by airshuttle and it offers you the possibility to take your car with you on board. There are three mayor shipping companies than serve the two islands and other destinations. They are: Balearia, Trasmediterranea and Iscomar.

For travel between Balearic Islands, I would consider Balearia to be the leader with the best network routes and the fastest ships. Trasmediterranea which now belongs to Acciona features super fast ferry links too, but they have a larger network extending all the way to the south of Spain, even serving Morocco, Algeria, Italy and the Canary Islands, which means that their frequency of service between the Balearic Islands is lower. Iscomar again serves similar routes to those of Balearia, but they have no ferry in service that can compete in terms of cruising speed with the fast and super fast Balearia ships. Then again, trips on a Iscomar ferry are usually cheapest, which may be important if you take your vehicle with you. 

Booking a trip is fairly simple, all of the three ferry companies have a reservation application on their websites and you can usually pay by credit card easily. Alternatively you could go to a travel agent where you can get the tickets instantly. There is one important thing you should remember. Before boarding in the port, you will be asked for a copy of your ID card or passport. Balearia does so and I would want to believe that Trasmediterranea and Iscomar do the same. There is usually no place where you can produce a copy, so you are well advised to have a copy on you.

Travel is quite uncomplicated. If you show up 30 minutes before departure with a valid ticket you will be fine. Boarding is usually pretty fluent and in case of Balearia there are no specific seats assigned, just the class you travel in. Seats are usually comfortable with plenty of space for your legs and you can walk around whenever you like. There are usually snack bars or even restaurants on board and with most of the larger ships, cruising is very quiet.

Ferry services are a real alternative to air shuttle links between the Balearic Islands. If you plan to travel to Barcelona or Valencia though, you might be better off taking the airplane, if your stay is going to be a day or two only.