Thunder Sword is a combo ability available in Kirby: Squeak Squad. Kirby gets this by combining the Spark and Sword abilities. It is similar to the regular Sword ability but has more range and it has the element of electricity equipped with it, meaning it can send sparks over water and electrocute/slash enemies at the same time. It turns into a normal sword underwater, but when Kirby jumps out of the water, it turns back into an electrified sword.
Curiously, along with the Fire Sword and Ice Sword abilities, the flavor text says it can't be used in water, when the truth is that it turns into a normal sword underwater. It most likely means that its electrical properties cannot be used underwater.