This is where almost all U-boat sims have been wrong, you could dive within 30-45 seconds without 'decks awas', with an alert crew. But you would
not dive for an attack at night. As Doenitz said,
"At night go in surfaced, and get close. They do not see you!".
Destroyers, escorts and corvettes would be seen long before the latter spotted the boat, so evasion from faster ones and running out slow corvettes on the surface was easy. Also a much better situation awareness, of course.
Until 1943 not all escorts had radar, also it often didn't work at all or badly, for various reasons. After this and with several escorts having radar able to triangulate, things of course changed dramatically.
From the very first sims to SH5, up to 1942 they would almost always see you. The best is still SH3 with the GWX or LSH mods..
In SH5 two destroyers found me surfaced in 1940, in rain and dense fog, at night, visibility zero. With 'found' i mean they followed and rammed. Then third mission or so sink five troop transports to get on. Adding one-eyed pirates as a crew and cheesy non-nazis all over the boat. That is basically when i stopped playing and got back to SH3
