First board: Focus on getting the flow of water to the exit, as fast as possible. Don't waste your moves on anything else. 700 points in gold-striped blocks.
Second board: Make combos from any side just to get the water to the key and the exit as soon as possible! This board can be really hard; if you don't have a direct match under the key you probably will never get one... 5,600 points in gold-striped blocks.
Third board: Just match some combos to get the water to the exit. Use your Color Burst to help you! 25,000 points in gold-striped blocks.
Fourth board: As target score is not that high for a score level, you should be able to easily pass the level if you've reached this board, the Treasure Room.