The fact that Shanks lost his arm has nothing to do with his strength. Shanks sacrificed his arm for the New Generation of Pirates. We can make out that he was strong from the start of the Series itself.
He had a large crew with many monsters in them like Benn Beckman and Lucky Roo. So to gather such a large crew he did require strength and by that we can see he is not an ordinary pirate.
It is not that he could not dodge the attack of the Sea King. He could have used Haki to just escape its attack.