I was on the Freedom last October. Yes, the lines are usually pretty long for the FlowRider. Check your cruise compass for the days and times it will be open. Also, there are both Boogie Boarding (or body boarding) and stand up surfing sessions. Again, check your cruise compass to see whether it is a surfing or boarding session. The way it works is they will let you ride the waves until you "wipe out," then you have to get out, and go back to the end of the line if you want to try again. The only exception might be if you wipe out within the first 10-20 seconds, then they may let you try again right away without getting back in line. The line I thought moved pretty quickly though.
Also, this is an activity that you have to sign a waiver for. Waivers will be on the same deck as the flow rider before you get in line.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas