Well outside of Kyogre, Blastoise is the fastest. Quickly followed by Jellicent who seems to be a little more useful in double battles ;)
meh I couldn't help it so I decided to give a little more info.
Since Kyogre is banned in smogon doubles you can already scrape him of the list. Blastoise is a very interesting and powerful Pokemon, however it's water spouts can easily be checked so I would not easily suggest using it in smogon doubles
Jellicent is a water type that works exceptionally well in smogon doubles and should also be the one that can use it the best in smogon doubles due to it being immune to Close Combat Hitmontop who seems to be a very common threat in the doubles metagame. However I will not suggest running a double scarruption with to countering types as they work well, but mostly in their respective weathers with Water Spout adoring rain and Eruption sun