In principle, you should aim your red laser just below where you see the blue fish.
Work Step by Step
Blue/violet light travels more slowly in water, compared to red light. This means that the blue light will refract and bend slightly more than red light will. So, in principle, a red laser fired back along the line of sight would not bend as much toward the normal/vertical, and it pass above the fish. So you should aim slightly below where you see the fish (though in practice, due to instrumental limitations, the difference is essentially negligible) .