When light passes through water, different wave lengths (hence different colors) are filtered out at different depths. Blue and violet penetrate to a greater depth than red or orange light. Even in clear water red and orange light only hit about 16 feet, yellow light around 33 feet and green light hits about 50 feet. That leaves only blue and violet pass the 50 foot mark. That's why water looks bluer the deeper you go down.
The white spectrum (6500k-20,000k).
Actinic emit a narrow spectrum of blue/purple and a small amount of UV
. Actinic's are not necessarily need if you use a high color temperature like 20,000k halide lamps for the fact that they produce the blueish light need for some photosynthesis.