The rule of thirds
Take a look at your previous beach photos. Is there a horizon line in the middle? How does it look to you? This actually slices the photo in half for observers, and is disconcerting.
Instead, stick to the rule of thirds. This means you should place the horizon in the bottom or top third of the beach picture instead.
Frame your beach picture
No, this isn't about the frame you buy later. This is about giving an anchor to your image. Look for natural frames for beach photos, such as a rocky outcropping or a leaning palm tree. You are looking for something that naturally places a frame around the target of your picture, the beach.
Get rid of bad lines
Before you shoot, really examine all areas of the image. Is there something distracting or ugly in your shot that you didn't intend to capture? Cars, electrical lines, a stray sunbather (who you don't intend to get in the picture, such as a topless sunbather in a family portrait) all can take a good picture and ruin it.
Also be sure your horizon line is straight. Sometimes, especially if an element in the foreground distracts you, you can end up with a lopsided beach.