Typically such zones are small and prone to interference, if you move just a little bit your coverage could disappear and reappear. The same happens in the 'urban canyon' of big cities between the high rises.

Even so, the default is that if you're in a hull you should not expect to be able to get out, and a submarine especially has a nearly cylindrical hull with only a very small hatch up top and this particular one has a few windows.