I use my Zodiac Pro 420 to go up to 13 miles offshore. However this is normally only when with other ribs., otherwise on my own i tend not to go anymore than 9 to 10.
Only ever go out that far when the forecast is perfect. Any sign that the wind is getting up then i'm on my way back in as i use the North Sea.
Latest trip was 60nm out next to the shipping lanes, (not in them) watching the tankers go past as its a very busy stretch of water going up to the Thames.
I also have flares, handheld radio, gps, compass, phone and have just ordered a fixed DSC VHF as these trips are becomming more frequent.