Slip-bobber rigs consist of a piece of thread tied into a knot on the fishing line, a glass or plastic bead, then the cork. The knot slides up and down to adjust the depth you’re fishing, the bead keeps the bobber from slipping over the knot and the cork slides freely up and down the line, stopping when it reaches the knot. When using slip bobbers in strong current, you must use enough weight to keep the bait down, otherwise the current will push the slip bobber through the line all the way back to the hook. It may take as much as an ounce when the current is really ripping, so make sure your slip cork is big enough to handle it. 

They may be a little more difficult to find than popping corks, but if you look hard enough, you can find slip bobbers in most coastal tackle shops. Those designed for saltwater fishing are usually much bigger and able to stand up to heavy duty.