Heavy Water

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.