Complex sentence: Heavy German gunfire sank or immobilised all the small boats that were to transport the Commandos back to England; the Commandos had to fight their way out through the town to try to escape overland.
Split into simpler sentences: Heavy German gunfire sunk or immobilised all the small boats that were to transport the Commandos back to England. This meant that they were cut off and had to fight their way out through the town to try to escape overland.