You've got the idea. Light travels through air MUCH better than it travels through water, and I believe salt water is even worse than fresh water. As for traveling through a vacuum, I'm not 100% sure. Star light is traveling through a near vacuum and it still fades with distance. But then space isn't a perfect vacuum and the distances are huge by comparison.
And certain spectrum of light travel better through salt water than others. Here is a chart that paints a pretty clear picture.
And here is a chart provided by Current Orbit which is very similar to the Fluval fixture you asked about. They use 0.3 to 0.5 watt leds like Fluval does.
Notice that at about halfway down in a 24" deep tank they get a PAR of 40 or less. Most corals in this hobby need more PAR than that. IMHO even zoas, softies and mushrooms need a PAR of 50 or higher just to stay healthy, let alone grow and propagate. My led fixture is 50" long and covers a 6' long tank. It uses just 88 leds, some are 3 watt and some are 5 watt. I run mine at 90% on 3w blue channels, 40% on 5w white and 3w violet and 10% on the few 3w green and red. With those settings I get a PAR of 190 at 12" deep in my tank. At 100% on all 6 channels (that's what Current Orbit used in their test) I get a PAR of 230. That's almost 6 times what the Current Orbit gets.
