From the examination she has inflammation due to drying her gills out too much, the equivalent of keeping an amphibian dry for too long. She should theoretically recover when going back into the water but doing a vigorous exercise with inflamed lungs (gills in her case) will often cause trouble breathing and hyperventilation. She's probably having an underwater coughing fit. Depending on the current she could be fine, but gondola's usually ride on slow waters even if it's next to a waterfall she should technically be fine as long as she's not directly under it. As a mermaid she should have her own version of a swim bladder so sinking shouldn't be possible but monster girls have weird anatomy anyways.