Since the sailaway party is so soon after getting onboard, and many people are still waiting for bags to arrive in staterooms/exploring the ship, I don't think the pool area and Aquaduck is as crowded as, say, your first sea day. For this reason, we totally packed all our swimwear in our carryons with the intention of hitting the pools right away, but we also got caught up in all the other happenings on the ship- so I would think swimming immediately is not on everyone's minds. I am sure it varies cruise to cruise, but doubt there are tons of people in line for the most part.
FWIW a long line is about an hour or more, which is what we encountered on our first sea day. Later at night and on days at port we found it to be 5 mins or less.