If you are referring to this style, they are the go to float on Lord Howe Island for garfish.
Like you said, you can adjust the weight by filling them with water. It's great for adding casting distance.
With gars you don't have to worry about balancing the float, like you do with blackfish, gars feed on the surface.
I prefer a more streamline quill or pencil style for blackfish, especially when you are fishing at a set depth, I think there is less resistance when the float goes down, and when the fish is towing it.
But I know it's working for you, so you aren't doing anything wrong.