Yes, they are definitely plants.
Back when they had the “sneak peek” of rehearsals for the show, whilst waiting to see it for the first time, I saw a staff member whom I recognised from earlier in the day at the Mermaid show in Shark Bay. He walked into a backstage area in uniform and came out a little while later with a couple of other staff members, however we was now in plain clothes.
He Spoke with the other staff before they left and went into the show stadium whilst he waited outside like all the other park guests. I figured he just may have finished his shift and was wanting to see the show?
Anyway he ended up being the one chosen as a volunteer, so I figured some sort of gag would occur, I initially thought he may fall in the water (like the plant did at the seal show at Underwater World/SeaLife).
I did see him again later in the day back in uniform near the park entrance.