What is the fastest marine mammal?
This article by R. Aiden Martin doesn't have citations, but is a great read with a lot of detail on observations and mechanics of animals moving in water. If you trust the numbers the author gives, the top 10 marine mammals in terms of speed (in mph = 1.6 km/h) are:


  Dall's Porpoise (Phocaenoides dalli), leaping   34.5 mph
  
  Killer Whale (Orcinus orca)  34.5 mph
  
  Shortfin Pilot Whale (Globicephala macrorhynchus)   30.4 mph
  
  Blue Whale (Balaenopterus musculus) 29.76 mph
  
  Fin Whale (Balaenoptera physalus)   25.42 mph
  
  California Sea Lion (Zalophus californianus)    25 mph
  
  Pacific Spotted Dolphin (Stenella attenuata)    24.7 mph
  
  Common Dolphin (Delphinus delphis)  23.6 mph
  
  Bottlenose Dolphin (Tursiops truncatus) 17 mph
  
  Pacific Whitesided Dolphin (Lagenorhynchus obliquidens) 17 mph


As noted in the article, higher estimates of speed for, for instance, the common dolphin may be in fact records of them 'surfing' on the bow wave of boats (not propelling themselves through the water).