Refraction in this case is referring to the fact that when a beam of light goes from a substance of one density to a substance of another density the speed of light changes and a beam of light will bend. To determine which way the light ray will bend, draw a normal line to the interface of the two different substances. In other words, a line perpendicular to the surface.
If the beam of light - your laser - is going from air to water, the density of the water is greater than that of air and the ray will bend toward the normal line. That means it will bend down toward the bottom of the water.
Because of this bending the fish will appear closer to you than it actually is so you need to aim farther out than where the fish appears to be. How much farther out depends on the angle at which you are looking at the fish. If you are looking straight down there is no bending, but as the angle of your eye away from the normal increases the more you will have to adjust your aim.