If a vessel is within one square of the Sonar Buoy it will be detected. If you are ever not sure what to do, first check the Data Center Screen located just below the Sea Strike logo. Click on the DONE button in the lower left corner of your Sonar Screen. If any of the Fleet have been detected by the Sonar Buoys, a Red Square will appear at its position. If no ships have been detected by Sonar Buoys, then you and the enemy take turns selecting points on the opposing Sonar Screen.
Blue Squares indicate empty ocean.
Red Squares are enemy vessels. Click to fire. Because the enemy vessels are of different sizes and strengths, the number of hits required to sink each one differs as follows.