I like the method mentioned by GreenPat, sounds like it should work fine.
I've launched a kite from a 30ft sailboat several times. A sailboat has an advantage over a powerboat because the keel gives you some control over direction - the wind is less likely to blow the boat downwind as long as there is some forward movement.
The method I used was to:
1) Lower the jib and set the boat on a slow beam reach under mainsail only (jib only could work too)
2) Inflate the kite on the rear deck, and have a friend hold it off the back of the boat
3) Unwind the lines off the back of the boat into the water, keeping them separated so they don't form a rats-nest
4) Bring in the lines one at a time and attach to kite
5) Get ready to lower mainsail, attach a leash, and jump into water with bar
6) Drop mainsail, and steer boat on a beam reach, speed should be very slow at this point
7) It's best if boat stops just before the lines tighten
Swim to tighten lines, check them for twists, and launch kite.
This has a few potential problem areas. There is the possibility of tangling the lines, although I have avoided this problem by keeping the boat speed very low, and keeping the lines widely separated. You also have a tricky situation if the boat is moving when the lines get tight - either the person on the boat lets go (giving you a live kite with potentially tangled lines), or you have to drag while the lines are checked and the kite launched (could damage the kite doing this).
In any case, I would not hook in until the kite was flying and checked out. And I would use a leash, plus check to make sure it can be released under load. Better to lose a kite than your life...