WiFi can be purchased on the ship in packages, but the packages are expensive, the MB go VERY quickly, and the quality is still so-so. If Internet is not a complete daily necessity for you, you can save money by buying the smallest package for must-have usage, then wait for a port day. Find a quiet restaurant or Internet cafe away from the touristy areas, and enjoy authentic food while getting to benefit from free WiFi.
Still, it's nice to unplug for a few days, so if you're able to minimize your Internet usage, definitely do that! Apart from people using their phones as cameras, I noticed a significant decrease in phone usage throughout the trip, and it was kind of nice to see people looking around them and at each other rather than down.