It depends on which day you are going to Ohana's, during your stay Wishes is schedule as follows:
2/11-10:00
2/12- 9:00
2/13- 9:00
2/14 8:00
2/15- 8:00
2/16- 9:00
2/17-10:00
2/18- 10:00
The water pageant goes by the Poly around 9:00, so you'd have to decide which day you were going to decide if you could see both from Ohana. If you go on a night when Wishes is at 9:00 I would say a reservation for 7:30-7:45 would be a good idea. If Wishes is at 8:00 then go for a 7:00 reservation and watch the water pageant from the beach.