I think this is a very interesting question. I can't see why a fish would not suffer the same fate as a human when lased in the eye - blindness. It seems that if using one near any creature (except perhaps those without eyes, and maybe potatoes), extreme care should be taken so that no harm is done. That means underwater that it should be shone only at non-reflective surfaces that you can see, so that it doesn't get some whale in the distance.
I think that just as visibility is usually limited underwater, the laser will be attenuated, and little visual harm will come to a distant thing that is invisible to the one with the laser. However, little is not none, and also remember that green light will travel farther in sea water than other colours too.
Play nice.