ship big
... Okay, I won't leave it there. If the ship grounded (as I believe it did) the actual impact would have been pretty minor. By "grounded," I obviously don't mean she rode up out of the water, but simply there was a submerged shelf of ice that she slid over, doing some fairly minor damage in the critically wrong areas. It'd be like a very small earthquake, if you were sleepy, you mightn't even distinguish it from the vibration of the engines that have been your life for the past four days.
There is a superb Titanic Channel episode about it with Parks Stephenson, if you're interested in learning more about this theory.