Hook knives are good, particularly if you mount them in a place which you can reach easily. However you attach it you need some assurance that it will break away if caught by a kite line without excessive force.** Securing with weak pull ties is one approach. I wear mine on my right seat harness leg strap. Other folks prefer serrated knives to better tackle leader lines. Some carry both as there have been some cases of kiters being tied up and having just one arm free to cut out of things. Also, it is easy to drop these knives in an emergency or not, as well.
From:
http://tinyurl.com/5b5d3t
** This fellow used a "soft" carabiner to attach his knife. It captured some kite line, the kite relaunched and looped him along at speed for a good distance until a panel tore in his kite depowering it. It could have just as easily dragged him on to shore under different circumstances.
http://fksa.org/showthread.php?t=516