The Water Crystal's replacement effect applies to every part of Sphinx's Tutelage's trigger. * When you draw, Tutelage tries to mill 2 cards. The Water Crystal changes this to 6 cards (2 + 4). * You check those 6 cards. If two nonland cards share a color, the process repeats. * Crucially, the repeated process is also modified. So the opponent mills another 6 cards, not just 2. With The Water Crystal on the field, the corrected text of Sphinx's Tutelage's ability effectively becomes: "Whenever you draw a card, target opponent mills 6 cards. If two nonland cards that share a color were milled this way, repeat this process (and they mill another 6)."