There's no magic, here.  If there is physically no way to get to them (even if you have to do it by "feel" only... which is common place on a boat, by the way), then as mentioned, creating an access hole is the only way.  Don't worry - this is a common practice and if it's done properly will have no ill effect on the boat.  Tip... use a bigger sized access hole than you would originally think - might as well make your job as easy as possible.  Obviously, the size depends on the area - and I'm not going to recommend a specific size - just that it's good to go bigger.  If you're not comfortable with this project, don't be afraid to hire this out.