Complex sentence: Churchill's design caught the attention of the US Navy, which wanted to issue it to their underwater demolition teams in the early 1940s and fins of his design can still be found in just about any sporting goods store or surf shop.
Split into simpler sentences: Churchill's design caught the attention of the US Navy, issued it to their underwater demolition teams in the early 1940s. Fins of this design can still be found in just about any sporting goods store or surf shop.