As far as the distress button that would be the DSC function. For the coast guard to know where you are at the radio needs to be hooked up to your GPS so that the DSC funtion will send your coordinates to the Coast Guard. You can register free through Boat US. See the following link for more info... I am no expert on electronics and others can provide you with more detailed info, however it appears that the one that BOE has listed here on their site for $99 is a pretty good deal. I will probably purchase that one as a back up in the next couple of days...
http://www.boatus.com/mmsi/