Part of the convention Always: clear weather and fog alike
Keep a proper look-out by sight, by hearing and by all available means, so as to make a full appraisal of the situation and the risk of collision. Radar and AIS do not replace a pair of eyes at the window, they are added to them.
At sea it looks like this
An officer buried in the radar display missed a small yacht with no radar reflector alongside. That is a breach of rule 5, the rule most often cited in collision investigations.
The rule says«Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and the risk of collision.»