It actually has 2 lines, one is minimum, the other is maximum. I think it's fine to submerge all the way too since I've read most people do that.
I actually did go out an get a 50W since this heater just doesn't seem to be doing the job. The problem is that the 50W Elite has this thing that you're suppose to hang over the aquarium rim, and my tank has this plastic ledge all around so I don't know how that'll work.
Another thing I was thinking about is using my 10W with the 25W and adjusting the 25W, but I think the temperature would be too unpredictable and changing all the time since the 10W isn't adjustable.