Especially in clear water conditions where you know the first light bite only lasts less than an hour, don't be afraid of fishing in the dark - just do it carefully. I have hooked several fish on glow-in-the-dark corkies or spin-n-glows, and have seen other anglers catch fish this way. Back in June while bar fishing for sockeye on the skagit river with sand shrimp, we would use glow scent from the moment of the first legal cast, which at that time of year is about 4:15 am. Fish were often hooked in the first 5-10 minutes after the legal first cast. Fish can see much better in the dark than you or I - there isn't much light in their normal cruising depths in the ocean.
If you can't see your float, try other techniques like tossing a dick nite spoon with a dropper weight. They make at least one glow model of that spoon. I have also seen coho hooked with spinners in frog water well before I could see a float.
As to the original question - I don't know why using a glow light on a float would be illegal - you are clearly not using the light to attract fish, because you don't want them to attack the float - you want them to bite the business end.