You've asked a big set of questions with a range of parameters. Generally if you check out the contents of sensors.dat you can find the answers.
Have a look at this thread which deals mostly with radar and sonar sensors but is pertinent to visual as well.
http://www.subsim.com/radioroom/showthread.php?t=103709
Running decks awash I've been able to pass to within 1000m of a DD in 1940 in bad weather, but have also had my scope seen at 1800m in calm weather in the same year. A lot depends on the quality of the crew also. Elite DD's later in the war will pick you up on radar much earlier than they will see you anyway.