It seems you already know this, but I think I should post this anyway, just in case.

Confidence is important when it comes to lucid dreaming. When you repeat to yourself that you will have a lucid dream, don't just say something like "I will have a lucid dream" without really thinking about what you are saying. Really believe that you will. Convince yourself that you are going to have a lucid dream tonight. Being confident that you will have a lucid dream will greatly increase your chances of having one.

I'm not saying you are doing anything wrong though. Having dreamt of waves after focusing on the sea for the first time sounds like a great start to me.

Keep it up, and I promise you will be lucid dreaming in no time.