Was on Freedom with one FlowRider and Oasis with two. Yes on sea days the lines will be long but my daughter and I never waited longer than 20 -25 mins at the worst. They will divide the FlowRider in half when really busy to move things along...there is still enough space to make a complete fool of yourself. Actually the boogie boarding is real easy and the staff will give you tips to survive your first ride.
My opinion is that the best FLowRider times is when the cruise is in port. Just check the schedule. On Oasis when we were in Nassua last August we returned to the ship early (1:00ish) and had the entire FlowRider to ourselves for about 30 mins. We actually got burned out on it and after awhile my 43 year old body said enough. Another tip is to make sure you sign all your waivers online in advance. This will save time even though you still have to get a wrist band. Have fun.....keep your knees bent and stay low if your trying surfing!