1. Lewis is the Merman.

2. Lewis became a Merman on Valentines Day (second full moon according to my estimated time table).

3. Lewis is not affected by the moon, but is affected by something else.

4. Lewis's tail will be black, because he's a guy.

5. Lewis can touch water because A) he has more self control even then Emma. Or B) He prefected the water proof spray seen in S01E24 Love Potion #9.

6. Lewis's three powers will be...
Electric Eel energy (because it's a cool sea power).
Healing (because someone needs to have it if the series continues).
And Understanding Sea Creatures (which might have been Miss Chathams power in season 1. As she warns Cleo not to talk to certain fish, but never mentions the Freeze power).

7. We will see Lewis in three more episodes, Graduation + two.

8. Emma will appear in Graduation.

These are my guess's, I may be completely wrong but it's fun to see if one's right.