Complex sentence: The boats were not able to make this speed in bad weather, where they were usually wet and uncomfortable with cramped crew quarters, but they proved their toughness in serving through the Great War, despite being twenty years old.
Split into simpler sentences: The boats were not able to make this speed in bad weather, where they were usually wet and uncomfortable with cramped crew quarters. However, they proved their toughness while serving through the Great War, despite being twenty years old.