I want to program a dolphin that jumps and rotates like a real dolphin. Jumping is not the problem, but I don't know how to make the rotation. At the moment, my dolphin rotates a little weird. But I want that it rotates like a real dolphin does.

How can I improve the rotation?

public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
    Texture2D image, water;
    float Gravity = 5.0F;
    float Acceleration = 20.0F;
    Vector2 Position = new Vector2(1200,720);
    Vector2 Velocity;
    float rotation = 0;
    SpriteEffects flip;
    Vector2 Speed = new Vector2(0, 0);

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
        graphics.PreferredBackBufferWidth = 1280;
        graphics.PreferredBackBufferHeight = 720;
    }

    protected override void Initialize()
    {
        base.Initialize();
    }

    protected override void LoadContent()
    {   
        spriteBatch = new SpriteBatch(GraphicsDevice);
        image = Content.Load<Texture2D>("cartoondolphin");
        water = Content.Load<Texture2D>("background");
        flip = SpriteEffects.None;
    }

    protected override void Update(GameTime gameTime)
    {
        float VelocityX = 0f;
        float VelocityY = 0f;

        float time = (float)gameTime.ElapsedGameTime.TotalSeconds;
        KeyboardState kbState = Keyboard.GetState();
        if(kbState.IsKeyDown(Keys.Left)) 
        {
            rotation = 0;
            flip = SpriteEffects.None;
            VelocityX += -5f;
        } 

        if(kbState.IsKeyDown(Keys.Right)) 
        {
            rotation = 0;
            flip = SpriteEffects.FlipHorizontally;
            VelocityX += 5f;
        } 

        // jump if the dolphin is under water
        if(Position.Y >= 670)
        {
            if (kbState.IsKeyDown(Keys.A))
            {     
                if (flip == SpriteEffects.None)
                { 
                  rotation += 0.01f;
                  VelocityY += 40f;
                }
                else
                { 
                  rotation -= 0.01f;
                  VelocityY += 40f;
                }
            }
        } 
        else 
        {
            if (flip == SpriteEffects.None)
            {
              rotation -= 0.01f;
              VelocityY += -10f;
            }
            else
            {
              rotation += 0.01f;
              VelocityY += -10f;
            }
        }

        float deltaY = 0;
        float deltaX = 0;

        deltaY = Gravity * (float)gameTime.ElapsedGameTime.TotalSeconds;

        deltaX += VelocityX * (float)gameTime.ElapsedGameTime.TotalSeconds * Acceleration;
        deltaY += -VelocityY * (float)gameTime.ElapsedGameTime.TotalSeconds * Acceleration;

        Speed = new Vector2(Speed.X + deltaX, Speed.Y + deltaY);
        Position += Speed * (float)gameTime.ElapsedGameTime.TotalSeconds;
        Velocity.X = 0;

        if (Position.Y + image.Height/2 > graphics.PreferredBackBufferHeight)
            Position.Y = graphics.PreferredBackBufferHeight - image.Height/2;

        base.Update(gameTime);
    }

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);
        spriteBatch.Begin();
        spriteBatch.Draw(water, new Rectangle(0, graphics.PreferredBackBufferHeight -100, graphics.PreferredBackBufferWidth, 100), Color.White);
        spriteBatch.Draw(image, Position, null, Color.White, rotation, new Vector2(image.Width / 2, image.Height / 2), 1, flip, 1);
        spriteBatch.End();           

        base.Draw(gameTime);
    }
}

I changed my code a little. But I still have some trouble with the rotation. Here's the entire code. The dolphin looks at the wrong direction if I press the left or right key. For example, it looks down if I press the left key. What is wrong with the rotation? At the beginning, the dolphin looks at the left side, but after I pressed a key it just looks down or up.

I deleted the "rotation += 0.01f;" lines in the code. Is that correct?

    public class Game1 : Microsoft.Xna.Framework.Game
{
    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
    Texture2D image, water;
    float Gravity = 5.0F;
    float Acceleration = 20.0F;
    Vector2 Position = new Vector2(1200,720);
    Vector2 Velocity;
    float rotation = 0;
    SpriteEffects flip;
    Vector2 Speed = new Vector2(0, 0);

    Vector2 prevPos;

    public Game1()
    {
        graphics = new GraphicsDeviceManager(this);
        Content.RootDirectory = "Content";
        graphics.PreferredBackBufferWidth = 1280;
        graphics.PreferredBackBufferHeight = 720;
    }

    protected override void Initialize()
    {
        base.Initialize();
    }

    protected override void LoadContent()
    {   
        spriteBatch = new SpriteBatch(GraphicsDevice);
        image = Content.Load<Texture2D>("cartoondolphin");
        water = Content.Load<Texture2D>("background");
        flip = SpriteEffects.None;
    }

    protected override void Update(GameTime gameTime)
    {
        float VelocityX = 0f;
        float VelocityY = 0f;

        float time = (float)gameTime.ElapsedGameTime.TotalSeconds;
        KeyboardState kbState = Keyboard.GetState();
        if(kbState.IsKeyDown(Keys.Left)) 
        {
            flip = SpriteEffects.None;
            VelocityX += -5f;
        } 

        if(kbState.IsKeyDown(Keys.Right)) 
        {
            flip = SpriteEffects.FlipHorizontally;
            VelocityX += 5f;
        }

        rotation = (float)Math.Atan2(Position.X - prevPos.X, Position.Y - prevPos.Y);
        prevPos = Position;

        // jump if the dolphin is under water
        if(Position.Y >= 670)
        {
            if (kbState.IsKeyDown(Keys.A))
            {     
                if (flip == SpriteEffects.None)
                { 
                  VelocityY += 40f;
                }
                else
                { 
                  VelocityY += 40f;
                }
            }
        } 
        else 
        {
            if (flip == SpriteEffects.None)
            {
              VelocityY += -10f;
            }
            else
            {
              VelocityY += -10f;
            }
        }

        float deltaY = 0;
        float deltaX = 0;

        deltaY = Gravity * (float)gameTime.ElapsedGameTime.TotalSeconds;

        deltaX += VelocityX * (float)gameTime.ElapsedGameTime.TotalSeconds * Acceleration;
        deltaY += -VelocityY * (float)gameTime.ElapsedGameTime.TotalSeconds * Acceleration;

        Speed = new Vector2(Speed.X + deltaX, Speed.Y + deltaY);

        Position += Speed * (float)gameTime.ElapsedGameTime.TotalSeconds;

        Velocity.X = 0;         

        if (Position.Y + image.Height/2 > graphics.PreferredBackBufferHeight)
            Position.Y = graphics.PreferredBackBufferHeight - image.Height/2;

        base.Update(gameTime);
    }

    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);
        spriteBatch.Begin();
        spriteBatch.Draw(water, new Rectangle(0, graphics.PreferredBackBufferHeight -100, graphics.PreferredBackBufferWidth, 100), Color.White);
        spriteBatch.Draw(image, Position, null, Color.White, rotation, new Vector2(image.Width / 2, image.Height / 2), 1, flip, 1);
        spriteBatch.End();           

        base.Draw(gameTime);
    }
}