I wouldn't say it's hard to believe. It's more of a photo that will make you think for a moment when you see it.
In Mexico, there's a diving spot called Cenote Angelita. It's an underwater sinkhole, but what makes it different from other sinkholes is that there's an underwater river flowing 30 meters below the surface. This river, where salt water meets fresh water, is covered in a thick cloud of hydrogen sulfide, which would kill you if you tried to breathe it.
From above, it looks like a calm pool of water, but as soon as you dive to about 30 meters, you are surrounded by the misty river flowing gently through the clear water. The hydrogen sulfide cloud is over 3 meters thick, and it is extremely difficult to dive through.
A renowned photographer, Benjamin Von Wong, took breathtaking photos in the cenote; I've included his work below. Of course, it was edited, and there was a team of divers to provide the model with air between shots, but the props weren't Photoshopped, and everything you see in the photo was indeed photographed there, underwater.