GoPros are pretty good at filming underwater. They are also excellent at capturing the sad demise of remote-controlled aircraft. So it makes sense that if you crash a RC plane into the sea, you're going to end up with some awesome footage.
The video comes courtesy of an Australian RC plane pilot who, whilst out for a pleasant day's remoting, got disorientated by the sun and steered his poor plane, complete with GoPro, into the sea. That's a pretty good video in and of itself — expensive electronics crashing into the sea! — but it gets better under the surface. Because this is Australia, all of which is filled with incredible wildlife*, there's obviously pretty fish and turtles and stuff swimming around the downed plane.