"Realistic Sensors" just means that your boat is basically deaf, dumb and blind (kid sure plays a mean pinball - sorry, won't happen again)... but that setting, when selected, just makes the ranges that you can hear with the sonar, or see with the radar shorter. It doesn't restrict any acquisitions you make, so there should be an improvement in any upgrade, whether you have that selected or not. All the tick box does is set a multiplier factor, such that if you have it selected, the multiplier is 1x, so if the sonar is set to hear out to say 12,500 yards, that's what it is. When you de-select it, the multiplier might be 1.2, so now your sonar hears out to 15000 yards. Your sonar gets "upgraded", and is now good to 15,000 at the 1x setting, but is now good out to 18,000 yards with the tick box de-selected...
These figures in no way insinuates what you, the user, might encounter in your game play, and is no guarantee of improved game play... 
