For Sail-a-Wave on the Wonder two weeks ago, we watched near the stage on Deck 9. It wasn't terribly crowded, and they had reserved the pool area (with the cover over it, obviously) for kids. So we kind of hung back (like where the pool chairs normally are) and let my daughter go stand on the pool cover with the other kids.
It's worth noting that this only worked because they didn't do the runway/fashion show. According to David from the entertainment staff, they stopped doing that version of the Sail-a-Wave a couple months ago.