For what it's worth, I used this double-bladed hook knife that came with my harness:
http://www.dakine.com/p/kite/accessorie ... hook-knifeIt required a surprising amount of tension/pull to cut single lines when I was safely sitting in 3rd Ave mud and it seemed absolutely useless when applied blindly to a tangle of lines under water.
I bought these medical scissors, but this is
not a recommendation:
http://www.amazon.com/Prestige-Medical- ... 002WJHEHY/I received them today and they look like they'd do the trick (sturdy, serrated blades and MATTE BLACK!!!), but they don't work either: they bend easily, it took me 3-5 cutting motions to cut a single line and they completely twisted out of shape when applied to multiple lines. I can't imagine they'd cut a belt, as claimed in the first Amazon review...
@nexus, have you actually tried your scissors on kite lines? On Amazon, they all promise to "cut seatbelts like butter", but I doubt that mine does. It seems that @sflinux's dive knife with a blunt tip might be the most effective option, although I'd worry about cutting myself in a real emergency on the water.
Having had this scary experience in very mild conditions (close to shore at 3rd with little wind), I am now keen on finding the best available cutting tool, but at the same time I'm not sure it would save me in a more serious situation.
Lesson learned: stay away from lines in the water at all costs!