Coastal operations? Diesel electrics spanned the world in WW2.
But they could only make decent speed running on the surface, where the diesels could breathe. They had to constantly be on the lookout for enemy aircraft, and dive before they were spotted. I suspect modern satellite imaging would make them sitting ducks in open water.
Yes, that's true, and they could theoretically span the world now. That said, it requires much more in the way of logistics--getting fuel to run those diesels. Plus, the need to run those diesels periodically makes the sub vulnerable and on the surface.
And while technically possible to go global, there's a much bigger impact on the crew, working on a D/E sub on such a basis. There are less "amenities" like fresh water available, perhaps video games and TV, eating conditions and the quality of food are less.