I have tried, with various levels of success, to get underwater shots.
Secret #1: clear water.
Secret #2: Slow water. Air bubbles = bad pictures.
That's a tough combination to get. I mean, lets say the stream is perfectly clear, but has a fair bit of current. So you pick a slow area to do your pics. Now, you wade in, and your foot just kicked up a bunch of dirt, coloring the water there. And with no current, it doesn't go away.
So a slow, smooth current, and takes pics UPSTREAM of you. Or, stand on a rock and reach down in, without wading.
After all that, it's tough to get the fish to behave. When I've attempted it, I leave it on the line and control it by grabbing the line. Then it's aim and pray.
Make sure it's cool water so you're not overly stressing the fish. And to it as quickly as possible. It's a lot harder than taking out of water pics. I find I use it more for scouting than for nice pics. Big hole, aim and shoot. Look later. You can see the undercuts and such that you can't see from above. Occasionally, even a trout hanging in there.