Wavers are Dial powered personal vehicles. Normally exclusive to the Sky Islands of the Grand Line.
The waver is best described as being similar to a jet ski. It's a small boat, large enough for two people to fit, that is propelled by air in the back. Normally, through use of what is called a Breath Dial, that can normally only be found on Sky Islands of the Grand Line. They have a wheel in the front that allows them to travel on land as well as water.
They are said to take ten years to learn how to ride one and the user always crashes on their first try.
Nami of the Straw Hat Pirates has her own personal waver that has a few exceptions that you can't find on most. Her waver is powered by a Jet Dial. This is similar to a Breath Dial but much more powerful and, at the time in Skypiea, thought extinct.
Though her waver was found by Luffy, the fact that she is the only person who can actually ride the Waver made it her's by default. No one except for Luffy has even tried riding it.
A Gift from Heaven
The name Waver was first heard when Montblanc Cricket handed Nami the captain's log of his ancestor Montblanc Norland. In it Norland spoke of an object he got from another person he met on his adventure in the Grand Line. When Nami first heard of what a Waver can do she was very interested in having one of her own. At the time, she didn't know she already had one on the Going Merry.
Nami got her Waver some time after the crew had left Alabasta and Nico Robin had just joined the crew. They were sailing on a clear day when out of nowhere a giant galleon called the Saint Bliss fell from the sky, upside down. It crash into the water, barely missing the Going Merry. To gather information on how to get to the Sky Island, Nami called for a salvage mission to find the clues that could lead to their goal. Luffy was among the salvage team that went to investigate and found the busted waver. It was among most of the junk they brought up from the galleon. None knew just what this was.
Eventually, crew made it to the sky island known as Skypiea and met a local named Conis. Her father, Pagaya is a dial ship engineer. He arrived riding one and Nami recognized it to be similar to the junked waver they found at the bottom of the sea. They presented it to Pagaya, and he attempted to fix it.
Luffy riding Pagaya's waver
Luffy was the first to test out Pagaya's waver and he wiped out almost immediately. This came as no surprise to Conis or her father. The waver takes many years to master. Conis had only just recently learned to ride it well. What came as a surprise to everyone was that Nami had the waver practically mastered on her first attempt. Her skills in navigating and her instincts the waves seemed to aid her.
After the crew had been kidnapped by Enel and in the process of their escape. Pagaya caught up to Nami and delivered her personal waver. Here he told her how he was surprised this waver had an extinct Jet Dial. This would make it far stronger then your normal waver.
Nami's repaired waver
After Zoro and Robin were defeated by Enel with ease it was Nami who relented to go with him. Her only request was to bring the waver. Later it was used to allow Nami and Usopp to escape Enel's Ark, Maxim.
Nami eventually met up with Luffy again and together they used the full power of the Jet Dial to ride up the collapsing Giant Jack. This allowed Luffy to get high enough to take a shot at Enel. With Enel defeated, Nami took the waver back down to the Blue Sea and was over joyed that it worked on water just as well as it did on the White Sea.
Sometime during the upgrades Franky had made on the new ship of the Straw Hat crew, the Thousand Sunny, Frank got around to altering Nami's waver for the Docking System he had developed. He was rather pleased with himself, but made the huge mistake of doing these changes without Nami's permission, and she was quite unhappy.
Shiro Mokuba I
Now calling the waver, Shiro MokubaI (White Rocking Horse). The alterations included a white horse figurehead and a new tread tire that would improve performance on land and double the speed of the original model. The light that was on the original waver is inside of the mouth of the horse figurehead.