That's awesome - I've never seen a picture that showed the air bubble like that. Nice job! It looks like a Rhantus sp. with that shoulder spot.
Thank you! I looked up Rhantus
and it does look a lot like it. However, someone else suggested that they see a spike on the ventral surface running from the thorax to abdomen. And that therefore it is a scavenger beetle, family Hydrophilidae.
I am no expert so I am a little confused now. I am going to try and get a good photo of the ventral surface and post it for comment...