ok there are several dangers to falling
1) have one foot in and one foot out of the straps
If you have the board one foot in and one foot off, kick the board off that's the most important thing to remember
2) landing feet first when you crash.
Don't do this. Even if the water is deep you never know if there's some log or obstacle under the water. don't land feet first. You want to ball up, not a complete ball just a little bit compact, hands in front of your chest, legs bent. You want to hit the water with your butt or body and allow your whole body to absorb the impact. Landing on your feet in shallow water can break an ankle.
3) Holding onto the bar with a death grip.
Don't do this. Let go of the bar entirely. It allows you to ball up a little bit and take a better impact. If you hold onto the bar with a death grip you are likely to loop the kite by accident. Just let go of the control bar and absorb the impact of the water.
Relax. Kick off the board. Ball up a little bit but not too hard (you want to be relaxed not tense when you hit the water). Don't fight it just allow the impact to happen.
I mean it's pretty much the same stuff that happens when you snowboard or skateboard. You don't want to be all arms and legs flailing around when you crash. It does look pretty awesome though on a video when someone is just flailing their arms and legs wildly so . . . yeah if you are taking a good crash video go out but if you want to crash safely just relax and absorb the impact with your body not your limbs.
Basically I'm saying the same thing everyone above me said. It's basically the same stuff you were taught while skiing with the one addition of make sure you don't land one foot in straps one foot out.