All of the above
Assigning crew members to maintain a lookout in all directions
Remembering to regularly look astern for overtaking vessels
Being aware of blind spots caused by sails, spray-hoods, dodgers etc.
Being especially vigilant when raising and lowering sails, attaching fenders and warps, entering and leaving harbours and marinas.
Listening out for sound signals
Using RADAR (if available) in poor visibility
Your vessel's maneuverability and stopping distance
The state of wind, tide, current and proximity of navigational hazards
Your vessels draft in relation to the depth of water
At night, shore lights in the background that can make it difficult to see other vessel's navigation lights
The prevailing speed-limit