Depending on how close up you're shooting, even f/3.5 may not be enough. Granted, when I'm shooting fish, I'm usually using my 100mm macro, but I seldom go wider than f/9. That's when that off-camera flash comes in very handy, because now light is no longer an issue. Top down shots can be awesome, but they're also difficult. The first thing you need is some sort of waterproof device for your lens (I made a box out of acrylic with an open top, the four sides painted black and a clear bottom). Basically, you need a way to get your lens below the surface of the water, or the reflection of the water will prevent you from taking a clear picture.