Sea Snake going up for a breath.
Diving in Australian waters, you frequently see them going up to the surface to breathe. Then then descend, sometimes right to you. I've had them bang into my mask, perhaps seeing their own reflection. They get in the way of photography and you sometimes have to just pick them up and move them aside. Why so blue?? Below about 30ft of water, all the red, yellow and green spectrum light is absorbed by the sea water and everything looks blue unless the diver provides a really strong light source..