For your nova, turn it down as far as it will go. Many bowfishers are shooting compounds set less than 30#. More than 50# is just gonna give you trouble in shallow water. So if your bow won't go less than that, I'd suggest getting a different bow.
Aim depends on depth and distance. If they are close to the surface or close to straight down aim right at them. As they get deeper and/or father away, aim lower.

A better way to say it is... When you start shooting aim low. When you miss, aim lower. Keep shooting.