Oil Slick on Lake Eats Teens

by Kobe
(Finland)

Trying to remember the name of this 80's film or horror short:

Some kids go to the lake. In the middle of the lake there's a wooden platform and they all swim there. They can see in the water some sort of black oil slick (?) that seems to be following them.

It catches them one by one and pulls them under the water. In the end, the camera pans back to the beach and hiden in bush there's a warning sign that reads 'Don't Swim'.

-Kobe