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This ability causes the move Techno Blast to become Water-type. It does not change it in any other way, however, and has no effect on a Pokemon without this move.
Only two Pokemon in the game can use Techno Blast, and the only one who can actually make use of it is Genesect. However, you really shouldn't be using this item even on Genesect, for a number of reasons. For one thing, holding Douse Drive means that Genesect cannot hold a boosting item, and will miss the kick from Life Orb or Choice Scarf dreadfully. While Genesect does not have any other Water-type coverage moves, Genesect has access to other coverage moves, such as Thunderbolt, Ice Beam, and Flamethrower, which also allow use of another boosting item - not to mention that a Life Orb boosted Hidden Power Water is stronger than a Douse Drive Techno Blast regardless. Like the other Drive items, you should never use Douse Drive competitively. It is just not worth it.
The Douse Drive is received in Pokemon White if you show Genesect to the Scientist inside the P2 Laboratory on Route 17. It is not obtainable in Pokemon Black.