The water team is very balanced and almost every cards has a part to play. Magic is obviously this splinter strongest asset. So if you are looking to build a strong magic team, it should include Medusa, Mermain and Pixxie, couple that with Alric for an extra boost. Lightning Dragon is currently the strongest Dragon card in the team. But do remember, to use it, you will need the Selenia Sky summoner.
I recommend not to combine the cards until Sunday, wait for the battles to start. There might be different format to play in.