Heavy Water

Transmutation [water]
Level: Artificer * 4, Cleric */Oracle * 4, Druid * 4, Hunter * 4, Ranger * 4
Components: V, S, DF
Casting Time: 1 Standard Action
Range: medium (100 ft. + 10 ft./level)
Area: cylinder of water (5-ft. radius/level, 30 ft. deep)
Duration: 1 minute/level (D)
Saving Throw: None, see text
Spell Resistance: no
  You cause a volume of water to become heavier than normal.
  Swimming in or through such water requires a Swim check with a DC equal to the saving throw DC of this spell; even creatures with a swim speed must attempt this check. Success allows a creature to swim at up to half its speed as a Full-Round Action; a creature cannot swim as a Move Action while in an area of heavy water. If a creature fails its Swim check by 4 or less, it makes no progress.
  If it fails by 5 or more, it goes underwater.
  All Perception checks to see through the affected water take a -10 penalty. All ships sailing through an area of heavy water move at half speed.