Caveat - not a Mac 25 owner.
Roland5048's observations make sense.
Roger MacGregor's focus was on boats that could be easily trailered. The compromises involved make MacGregors not very well suited to blue water sailing.
That said, I know of Mac 26M and X owners who sail from Florida to The Bahamas (a good deal more than 5 miles offshore). These boats are probably as or more tender than the 25. Naturally, they don't venture out unless they are confident that the weather won't get too nasty. I would say it doesn't matter so much how far from land you are going as how confident the weather and subsequent sea state will be amenable to the boat you are sailing.
A heavier boat with plenty of weight below the waterline will be safer and more forgiving than a lighter boat like the Mac 25. People have sailed around the world in smaller boats, so in my opinion it has a lot to do with how well you know how to handle your boat in adverse conditions. Think in terms of worst-case scenario, and know your boat inside and out before you venture out of sight of land.