Matthews says what he saw was "unnerving."
He described it for "Early Show" co-anchor Harry Smith.
"We did two dives," Matthews said. "We did one along an oil platform about 40 miles from the Deepwater Horizon (the rig that exploded and sank, causing the mammoth spill). We did another dive out in the open water about 1,000 feet deep, about 20 miles from the actual leak.
"On the platform dive, we didn't see fish for the first 30 feet. And that's rare. Oil platforms are really kind of towers of life. And when you splash down, typically, fish come up and check you out right when you hit the water. We dropped down about 30 feet before we saw anything. And the water at the top was very hazy, very murky. (That's) abnormal for water that deep and that far out.
"The second dive, once I got in the water, I could really not see very much at all. The visibility was terrible. In fact, on that dive, I'd planned on going to about 60 feet, but I determined it was too dangerous because the visibility was so poor and only went to about 15 feet.
"It was unnerving. … When I splashed in, my mask was covered with oil. I wiped it off as best I could. But I never really gained full comfort when I was under that spill. So I just decided -- and especially because I was by myself at that time, I decided not to take any unnecessary chances."
He added, "At every depth that I went to, up to about 60 feet, I saw at least signs of oil or I saw unnatural things that I typically don't see when I dive in the Gulf of Mexico. So all the debate about the plumes, I didn't see any massive plumes of oil. But I did see signs of oil, again, all up and down my dive.
"And I can tell that you as a matter of fact that we're not seeing all the oil from this leak when we fly over it in helicopters or go out in boats."
To see the entire interview and many of the pictures Matthews took documenting his dives, click on the video below: