Regular water just gets in the way in a Slasher Movie. Have you tried Offscreen Teleportation? The displacement shift just gives you away. And if your victims can swim, they get away with ease. If they can't, they may drown before you get to them, and that's no fun.
Not only does it not impede teleportation, it can sometimes teleport your victims. One second they fall from the docks, the next they've drifted a dozen yards away. Even when there are no currents.
Regardless of where your victim lands, this product has special properties. It can identify your victims, and keep them still and buoyant (even though they frantically splash around like someone drowning, and would likely sink or end up accidentally inhaling water if they did this in regular water), ensuring they will not reach safety, even when they are less than two feet away from that.
Nor will they see you as you are creeping up on them. This product will either make itself opaque on their end, or make sure they always look the wrong way.
This can apply to any body of water up to a mile in radius, even the middle of the ocean. Larger sizes can be bought in bulk.
Never go slashing without it!