You need water deep enough to turn the kite under water. A bit of current running away from you helps, but do not try in a strong current, you will lose your kite !
Usually a kite will try to head nose-down to the bottom. Don't panic! You can still feel your kite with your lines. Pull more on 1 line so the kite starts to turn up. If you see a wingtip pointing up, you're getting there. Let the wind and your pulling lift the wing upwards. You cannot fly out with one wing in the water easily, so you have to little by little, turn the kite around by pulling one line. As you get one wing out of the water, you have to switch sides that you're pulling, let the upper wing drop back towards the water, while pulling the lower wing up. Once you get the nose pointing straight up, an even, steady pull on both lines will raise the kite into the air.