That can be done in a number of ways, and almost seems like an art question.
You can use algorithms to manipulate pixels, like this (the concept applies to many languages and APIs, not just C++ with SDL).
You could use basic frames of an animation where you go from Frame 1 to Frame 2 to Frame 3, and so on.
It might even look fine just to have a static image move down the screen and then back up, depending on the style you are looking for.
Or you can use another method like playing a video (which can get more complicated, especially when you want your menus to appear underneath the waves).
That's assuming 2D. For 3D ocean waves, then you could google "OpenGL ocean wave simulation" (or DirextX if you're using that) where there are plenty of resources and discussions on many different methods.
When you say 'design', are you talking about how it should visually look, or how it should programmatically move?
You might have better luck creating a new thread about the art in the Visual Arts subforum, if you're asking how you can draw ocean waves or if you're asking about what art programs to use - those gentlemen would know better than I.
Otherwise, I just can't comprehend what you're asking. Maybe I'm just thick.
Is this a programming question?