I am not sure if I have had dry noodles before, but I can try and help you. If there is not enough sauce then I would immagine such a direct source of high heat is drying the noodles that are on the top. I would try and put more tomato sauce on the pasta so that it will stay moist in the oven. I would not thin out tomato sauce with water, because that is going to make it less concentrated and not add flavor, but make it less flavorful. I use chicken stock and or white wine to make more sauce then the original recipe yields. Of course my first choice is add more fresh tomatoes if I have them, or more canned tomatoes if I have them. Adding stock and or wine does make it less concentrated, but it is a much better choice than water because it adds flavor.
Maybe if you have a low or high setting on the broiler, turn it to low. I also might try not using the broiler. Just put it in the oven at 200 and that should melt the cheese in no time, without cooking the pasta too much.
I hope some of my suggestions help.