How to make joints not rotate using sprite kit or another way of making dynamic 2d water

Question

I want to creat a dynamic 2D water, i', following this unity tutorials

http://gamedevelopment.tutsplus.com/tutorials/creating-dynamic-2d-water-effects-in-unity--gamedev-14143

http://gamedevelopment.tutsplus.com/tutorials/make-a-splash-with-2d-water-effects--gamedev-236

And also this one

http://blog.prime31.com/water2d-part1/

Here is my code

class GameScene: SKScene, SKPhysicsContactDelegate {

    var box: SKSpriteNode!
    var nodes:[SKNode] = []

    override func didMoveToView(view: SKView) {
        /* Setup your scene here */

        self.anchorPoint = CGPointMake(0.5, 0.5)

        // Set physics body
        let borderBody: SKPhysicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame);
        borderBody.friction = 0.0;
        self.physicsBody = borderBody;

        // Set contact delegate
        self.physicsWorld.contactDelegate = self;

        box = SKSpriteNode(color: UIColor.blackColor(), size: CGSize(width: 200, height: 20))
        box.position = CGPointMake(0, 0)
        box.physicsBody = SKPhysicsBody(rectangleOfSize: box.size)
        box.physicsBody!.dynamic = false

        self.addChild(box)

        let one = SKSpriteNode(color: UIColor.greenColor(), size: CGSize(width: 20, height: 20))
        one.position = CGPointMake(box.position.x - box.frame.size.width/2, box.position.y + box.frame.size.height * 2)

        one.physicsBody = SKPhysicsBody(rectangleOfSize: one.size)
        one.physicsBody!.allowsRotation = false
        self.addChild(one)
        nodes.append(one)
        self.attachPoint(box, point2: one, box: true)

        let two = SKSpriteNode(color: UIColor.greenColor(), size: CGSize(width: 20, height: 20))
        two.position = CGPointMake(box.position.x, box.position.y + box.frame.size.height * 2)

        two.physicsBody = SKPhysicsBody(rectangleOfSize: two.size)
        two.physicsBody!.allowsRotation = false
        self.addChild(two)
        nodes.append(two)
        self.attachPoint(box, point2: two, box: true)

        let three = SKSpriteNode(color: UIColor.greenColor(), size: CGSize(width: 20, height: 20))
        three.position = CGPointMake(box.position.x + box.frame.size.width/2, box.position.y + box.frame.size.height * 2)

        three.physicsBody = SKPhysicsBody(rectangleOfSize: three.size)
        three.physicsBody!.allowsRotation = false
        self.addChild(three)
        nodes.append(three)
        self.attachPoint(box, point2: three, box: true)

        self.attachPoint(one, point2: two, box: false)
        self.attachPoint(two, point2: three, box: false)

    }

    func attachPoint(point1: SKSpriteNode, point2: SKSpriteNode, box: Bool){

        if(box == true){

            let newPoint1 = CGPointMake(self.frame.size.width/2 + point2.position.x, self.frame.size.height/2  + point1.position.y)

            let newPoint2 = CGPointMake(self.frame.size.width/2 + point2.position.x, self.frame.size.height/2  + point2.position.y)

            // create a joint between two bodies
            let joint: SKPhysicsJointSpring = SKPhysicsJointSpring.jointWithBodyA(point1.physicsBody, bodyB: point2.physicsBody, anchorA: newPoint1, anchorB: newPoint2)

            joint.damping = 2.0
            joint.frequency = 9.0;

            self.physicsWorld.addJoint(joint)
        } else {

        let newPoint1 = CGPointMake(self.frame.size.width/2 + point1.position.x, self.frame.size.height/2  + point1.position.y)

        let newPoint2 = CGPointMake(self.frame.size.width/2 + point2.position.x, self.frame.size.height/2  + point2.position.y)

        // create a joint between two bodies
        let joint: SKPhysicsJointSpring = SKPhysicsJointSpring.jointWithBodyA(point1.physicsBody, bodyB: point2.physicsBody, anchorA: newPoint1, anchorB: newPoint2)

        joint.damping = 2.0
        joint.frequency = 9.0;

        self.physicsWorld.addJoint(joint)

        }

    }

    override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
        /* Called when a touch begins */

        let node = nodes[2]
        node.physicsBody?.velocity.dy = 20000

    }

    override func update(currentTime: CFTimeInterval) {
        /* Called before each frame is rendered */
    }
}

Here is the result

This result without changes

After touch, execute code in touchesBegan method

After step 2, the nodes start to rotate and fall bellow the node

If they don't rate, following the the tutorials I will have to make SKShapeNode with the color of the node. But SKShapeNode is expensive, is there another way this effect can be accomplished?

The easiest way is to use image for the top part of the water and make it move back and forth, but it won't be dynamic..

If you have ever play Tiny Wings, the water there is implemented exactly the same way in the tutorials.

I don't know, maybe this can't be made in SpriteKit or I just don't know how