Complex sentence: A longer tube would place the lungs in deeper water where the surrounding water pressure is higher and the lungs would be unable to inflate when the snorkeller inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.
Split into simpler sentences: A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher. The lungs would then be unable to inflate when the snorkeller inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.