Water relaunching Low Aspect Ratio inflatable kites
You can relaunch an inflatable kite by getting it to a vertical crescent moon position, this is done by sliding the kite to the edge of the wind window and then launching it, by pulling the top line. You can easily get an inflatable kite in a vertical moon crescent position by pulling on one line and then suddenly on the other one. This is best used with low aspect ratio kites.

Water relaunching High Aspect Ratio inflatable kites
High aspect ratio inflatables are likely to crash in a leading edge down position and because of the large leading edge they will refuse getting in the vertical crescent moon position needed for launching the inflatable kite. There is only one way to relaunch a high aspect ratio inflatable and that is to get it on it's back by swimming towards it. In strong wind this is useless because the pull of the kite easily absorbes the distance you swim towards it. If that's the case you'll need to pull on the middle leader line of the kite and after pulling around 2 meters of the middle leader line, in an instant release it while holding on to the control bar. Your kite will immediately get on to its back.

Relaunching closed cell foil kites
If a kite crashes, it's leading edge is frequently down on the water. For relaunching a closed cell foil kite, you'll have to fly the kite in reverse. This is done by pulling on both brake lines hard, that is if you are using handles. If using a three line bar set with the center leader line connected to the trailing edge of the kite, you have to pull hard on the center leader line. Release the brake lines and steer the kite upward once it's out of the water.

These tips should help you if your kite falls while kitesurfing.