The first time I had ever seen this, i videoed the entire event as well from entry to exit. It was as if this species swims all the time. I sent it to a frogmouth expert and she had never seen that before. The bird chose to swim and flew away very easily after its swim. There appeared to be no explanation for the bird swimming as it was not particularly hot or anything unusual.
See photo. Eating Mocha Emerald. The emerald was hanging from a twig in a shaded area. It had been there for several minutes when apparently some small motion drew the attention of the robber fly. The dragonfly became aware of it's peril and tried to escape, but wasn't fast enough.