You could easily see it here, just click the Ice button and the Water button and look at both weaknesses and strengths.
Water, flying, dragon, rock, etc. It's just one of those pokemon that's weak to it's own type :) PS This is devoshi10 you might have met me before
Dewgong is ice and water type so it is good for beating dragon types, ground and rock types, plus flying types if I remember correctly.