I was on Allure in Sep. FANTASTIC! But to answer your question, yes you will see the Aqua show from the Boardwalk Balconies furthest aft. Not sure if you'll see all the show,but most of it. You'll also see the carousel, dancing water show, character show, Rita's Cantina, Johnny Rockets. The Boardwalk is, well, a boardwalk. Point is it my be somewhat noisy, and definitely not private on the balcony. But having said that, you will have some water view (unlike Central Park balconies) and lots to watch. Keep in mind the balconies are only large enough for 2 chairs. But there are other places to watch the shows, the best being from the seating. Order tickets early then if you want a different shoe time simply stand in line. If you can't get in you still have tickets. Also DO NOT MISS Blue Planet!!
Bottom line is don't book Boardwalk balcony to see the show, but if you like to people watch and it fits in, not a bad choice. Me.... I'm an ocean fan and like the outside balcony. For inside balcony, Central Park for quiet moments, but I think Boardwalk would be better view, just know that a lot goes on on the Boardwalk.