The sonar bearing lines are quite the handy tool for planing approaches, especially if you don't have radar yet installed. So don't discard them in the first place.
The "ship" icons in the stock versions appear when there is a visual contact. I am left to a guess at the exact values of visual ranges, but surfaced you have the highest visual range (my guess about 5 nm in top weather conditions) and submerged at periscope depth with your periscopes sticking out the water it is somewhat smaller (maybe 2 or 3 nm, again just a guess).
Visual range is heavily affected by weather conditions and day & night times probably factor in just as much. If you are cruising around in a pitchblack night in a storm with dense rain, you might not be able to visually identify anything more than 5 yards ahead of your bow.
I'm sure that the crews proficiency factors in (Deck Watch & Watchmen skill levels), but I personally noticed little difference between an experienced deck watch and freshmen at career start. (with empahsis on "noticed").
I'm also sure that there is a modifier and/or multipler somewhere in the code which could be altered/modded. If you do so, be careful as it might be the same modifier your targets use. Being visually spotted at 200nm by everything within that range, could ruin your day.
