I lke the idea, how the disciple of water allows it to use a mono deck wih much more potential, at the same time it makes hard to use in rainbow. But I think, the card right now is on one hand not fair, cause it would give mono water a huge advantage against all the other mono decks. On the other hand, lots of card would kind of loose their purpose and existing deck build would be killed.
This card would strenghten the mono deck, but it would not only be useless in the rainbow, it would also kill many duo or trio deck made with water. This is what i don't like...
The changed cards would be: Mind flayer, Chryasora, Toadfish (unupgraded) and Trident
Of course, it would take time to play a card and also play a disciple of water, but in a mono water, this would take about 2-3 rounds, after that you could qunint fairly cheap and destroy the towers of the opponent with trident, seems quiet strong to me.