A great pair of legs, a brain operating at full capacity and a good sense of humor.
Oh, you mean underwater. As a long-time solo diver and videographer, I'm very particular about who I dive with. I had too many pick-up buddies in the past (which resulted in an unacceptable rate of problems on dives).
I want buddies who are essentially self-reliant, but enjoy sharing the experience of the dive with me. They need to know where to position themselves relative to me not to interfere with my filming by flailing into the frame, stirring up sediment or frightening my subjects.
My occasional buddies include still photogaphers and videographers, rec divers and even a few spearos. They are all dive professionals.
If I'm diving without my camera (and therefore can be more attentive to the buddy relationship), I will on occasion dive with others including new divers. However, I rarely dive without the camera.