I wrote out a long description about bowfishing to help you, then I realized this post was made in 2017. You probably won't benefit from this now, but maybe someone else like me who comes along will.

Bowfishing is a growing sport and getting into it can be really frustrating, because there's a lot of missing before you start hitting. You don't use sights, you use reflexes. As others say, aim low. The deeper a fish is and the further away a fish is, the lower you have to aim. Based on this bowfishing guide you should aim 4 inches down for every 10 ft distance the fish is away from you. A good universal rule is just aim 6 inches below the fish.

If you're shooting big fish I would recommend having someone you are with get a back up shot on the fish. Even the best and strongest barbed bowfishing arrows can pull out of big fish when they take off. We try to get at least 2-3 shots in bigger fish before we try to bring them in.