The radar beam will bounce echoes off the sea around the ship, particularly if the weather is a little rough. This result will be a bright sunburst pattern in the middle of the screen which will be more pronounced in the upwind direction. You could reduce this by turning down the gain, the down side to that solution however, is that the echoes of more distant targets will be lost as well.
The solution is the sea clutter control. It works by reducing the receiver gain for a few microseconds after each pulse is transmitted, then gradually restores it to its former level. It works very well, but its use requires care. Too much sea clutter control will result in the loss of close range targets. At sea the sea clutter control must be continually monitored and adjusted.