Right now all planes in the fleet have wifi, except the older 300s and a few newer (used) 700s recently added. Southwest states that, hopefully, by the end of 2017 (after all the 300s are retired), the fleet will be 100% wifi enabled.
In my experience, while the wifi works great for the in-cabin entertainment system (with live TV), it unfortunately doesn't work so well as a paid ($8) connection to the internet.
