spotting distance
 

What is the spotting distance with normal weather/see for your enemy,when do you have to dive??

henny

This depends on your angle to the enemy... e.g. they can see you sooner if you have your side to them. As you present them with a larger silhouette.
And also the skill level of the crew on the ships.

You can test this easily..

Turn map contacts on...Start an historical mission... And the visual range ships can see is the cray/black circle around each contact on your map.