Place the potatoes in a large saucepan; add the water, seafood seasoning and salt. Bring to a boil. Reduce heat; cover and simmer for 25 minutes.
Add the shrimp; cover and cook for 5 minutes or until shrimp turn pink and potatoes are tender. Drain. Peel and dice potatoes. Peel and devein shrimp; cut into pieces.
In a large bowl, combine the potatoes, shrimp, onions and pimientos. In a small bowl, combine the mayonnaise, vinegar and sugar. Add to potato mixture; gently toss to coat. Season with additional salt. Cover and refrigerate for at least 1 hour before serving.