Surface signaling devices. If your navigation skills fail, you may surface far from the dive boat and you'll want to attract the crew's attention. To maintain buddy contact in the thickest soup, some divers hold the ends of a short rope. A longer rope will snag on rocks, plants, your leg, other divers, whatever. Hold the rope, but don't tie it or clip it.
Some divers prefer to just hold hands.
Knives and shears. The risk of entanglement in fishing lines and nets is greater, obviously, when you can't see the stuff. Shears are ideal, as you can use them with one hand on fishing line and small rope. Like headlights in fog, dive lights can sometimes help in murky water, sometimes hinder. Even if your light doesn't illuminate anything, it may make you more visible to your buddy.