The USCG, NOAA and the NWS all define coastal as being less than 50 nm from the nearest mainland or islands, and offshore as being more than 50 nm from the nearest mainland or islands. This definition works for me. It is official. 50 miles or more at sea you are on your own. The likelihood of getting service or rescue is minimal.
Equipment can fail.
The crew can get injured. The weather can deteriorate and seas can build to enormous proportions for days on end. Full responsibility for the preparation, repair, navigation and handling lies with the skipper and the crew he has chosen. Offshore sailing requires the highest standard of vessel preparation and skipper and crew and knowledge and skill levels. Most of us prefer to approach sailing on a learn by doing basis.