Idle speed is set by the PCM and is not adjustable. The screw you are fiddling with is just the stop screw to adjust the initial throttle plate position, it should not be used to attempt to adjust the idle.
Engine idle speeds are all computer controlled these days. Not like the old days when we had a screw to move in / out to adjust ourselves. There are a few things that can be done to "influence" idle speeds - see "Related Questions" below for more
== Is this engine the plastic cowling with idle knob and choke knob on the front pull it to choke and turn to raise idle? = Read more: http://www.justanswer.com/questions/1dpbt-25-hp-mercury-outboard-water#ixzz0HH1RenQM&B
Do you have a idle adjustment screw at the end of the throttle cable? If so then just back it off a little while the engine is running.Just take off the cover where the throttle cable goes, and adjust the idle by turning the screw.
Your vehicle has electronic fuel injection. As such, your idle setting is controlled by a computer (ECM) and cannot be adjusted in the manner I believe you seek. If you have an idle issue, there is most likely something else that needs attention in the system (dirty or faulty sensor, etc...).
You can not adjust the idle screw, it is sealed off at the factory. If you are having a high idle problem check for a vacuum leak or a bad idle air control valve. These will commonly cause a high idle.