On a scale of 1-10... a 4. Definitely better than no light when night fishing. However, on a couple of different night fishing trips, I have rigged my floating light (just like the one you described) out at one end of the boat, and my green submersible light at the other end. Each time, the green light ended up swarming with bait fish, and there wouldn't be a handful swimming around the white light.
That said, if I had not had the green light in the water, there may have been just as many flocking to the white light. However, they definitely seem to prefer the green light over the white.