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.