Actually I going by their movesets. Dewgong learns rest and aqua ring. Plus it better water type moves, like dive, aqua tail, and aqua jet, while walrein only learns water gun. I know that walrein learns rest too but that is beside the point.
You didn't do much research, obviously.
*Sigh*... Dewgong has a low attack, as well as special attack. If anything, Surf would be the strongest water move you should have named.
And Walrein learns Rest and Aqua Ring too? Okay... And Surf, Brine, Waterfall, Aqua Tail, et cetera. Water Gun is not the only water move Walrein learns. I don't know where you looked for information. Serebii's pokedex has everything.
Walrein > Dewgong. Stats and movepool show this. And with Hail, Walrein is a beast. Blizzard gets 100% accuracy, he recovers health, and doesn't get damaged. Yeah, Dewgong can learn Blizzard, but it doesn't have ice body.
I'm just going off facts, stats, and moves.