Hi Chris,
Photographs of seascapes can be incredibly captivating, particularly when you place the camera on a tripod and use slow shutter speeds to capture the swirl of the water. Of course, as you have discovered in the process of making this image, there are plenty of variables! Two of those are the tide and the sea conditions on the day. Trying to make a photograph like this when the sea is flat can be rather difficult, and yet at the sea is too rugged that can be just as challenging.
One consideration, though, is not to get too hung up on the shutter speed you set. In this photograph you used a 30-second exposure, but my hunch is that if you tried a shorter shutter speed of about 8 seconds (or even 4 seconds) the results could have been just as interesting. The risk in using too slow a shutter speed is that the image becomes too cloudy and you lose any impact in the water.
One of the other big variables is the actual overall exposure. In this image I have the feeling you've let too much light into the camera. If you were making this photo with the camera on an automatic setting, like Shutter Priority, then I'd look at using the exposure compensation dial to darken the overall image. To do this, set the exposure compensation to a minus (-) setting. In this instance, -1.0 Exposure Compensation might be enough to darken the overall image and bring more detail into the background.
The last of the variables in making any image is simply the composition. In this photograph I feel you've chopped off much the foreground. Given the chance, I would go back to this location and keep experimenting. As with most locations, the potential is here… it's just a case of discovering it.
Cheers, Anthony