1) If you opt to apply the underwater texture on the fish as a luminosity procedural, set the texture center so it's at the top of the fish, and adjust the Y falloff so it fades away at about the middle of the fish. This will keep the underside of the fish from being affected by the texture, and make it appear that the fish is being hit by refracted light from above.

2) You could apply the underwater texture to a large panel that's off camera. Then put a ray-traced light next to the panel so its light shines through it. Make the panel about 75% transparent, and turn on the Color Filter option. You'll also have to have ray-traced shadows on, and the panel must have its surface normal facing away from the light (or just turn on Double-Sided for that surface). This will really project the underwater texture onto the fish...and everything else that's in the light's view.