Further to Gorgo & others excellent advice, I would emphasize flattening the board when you do the water start and head downwind. Don't try and edge it until you have some good speed.
Whey you head downwind fast and the kite comes back up from your power stroke from the water start, you will also then find you then may "lose power" in the kite.
Two things you can do to correct this:
1. Keep flying (turning) the kite into the power zone and don't let it get to high (or low).
2. Edge the board so the kite pulls against it and moves forward in the wind window as you get going on the tack.
As per Gorgo, push the bar OUT if you have to much speed.
And look where you want to go, not at the kite.
Here are a couple of articles with more info:
Water start:
kitesurfing-handbook.peterskiteboarding.com/progression/water-startGoing upwind:
kitesurfing-handbook.peterskiteboarding.com/progression/going-upwind