Thanks for your support.
Now I realized I made an error when I dived with flooding.
I tested this in a single mission, and even though my bow compartment was almost completely flooded, the boat stayed on surface. When I dived to periscope depth, the boat begun sinking like a rock.
I should have stayed on the surface until flood was under control, and then dive. Maybe this is common knowledge, but I wasn't aware of it.