There once was a lagoon show that utilized jet skis and watercraft and stuff, but I forgot the name of it. It was just a quick summer promotion. However, I don't see a nighttime show coming to IoA anytime soon for several reasons.
1. Viewing Space
There are plenty of places from which to see the lagoon from IoA but none of them are very large spaces well-suited for holding several people to face one direction to look at one thing. The only exception to this is the area in front of the JP Discovery Center which currently holds the standby line to get into the WWOHP. Other than that, areas facing the lagoon are either non-linear (TL, SL), not wide enough to fit people (LC, PoE), or do not give expansive enough views of the whole lagoon (Marvel, WWOHP).
2. Crowding
US has plenty of wide open spaces for people to gather around to watch a nighttime show...IoA, not so much. The narrow paths make it hard for people to see the lagoon in large numbers. Since IoA draws in more people as it is, having everyone gather to the perimeter of the lagoon would seem like a logistical nightmare. Not to mention the complaints and anger people would feel once they go to a very limited-space spot that is already full, leaving them to trek all the way around to another island and gamble with viewing availability.
3. Rides
There's too much attraction interaction with the lagoon. A nighttime show would require shutting off the lights of any attraction touching the water, which includes the Burger Digs Restaurant (a Meal Deal Location), Mythos (premiere theme park restaurant), Me Ship, The Olive (this one could close early without complaints I think), and finally the High in the Sky Trolley and Hulk--even if the solution was to close these attractions early, how early would that be? Hulk and Trolley, on peak nights when this show would be feasible, easily draw up to an hour queues--do we really want to close those two headliners that early on?!
4. Creativity
What would this show be themed to? Classic movies seems like a no-brainer for the Studios but IoA is based on literature so there doesn't seem to be a natural fit. Now you could argue that they could do an adventure-themed show with watercraft and pyrotechnics and popular songs and stuff, but that would clash with the theming of some of the islands from which the show was to be viewed from, namely Harry Potter, which would shut down in an instant if it was mixed with rock music at night.
5. Repetition
There's already a nighttime multi-media show at USF. i think they have fountains, waterfall screens, and fireworks all in one show for the sake of covering the bases--they don't need two shows with the same elements occurring around the same time right next door to each other!
6. After the Show...
Though Illuminations offers a similar viewing area layout with everyone crowding around a circular lagoon, Epcot offers the expansive-enough walkways to support a good chunk of its capacity as they all leave the park simultaneously. IoA, not so much. It's tiny walkways have a hard enough time supporting people as they pass through, imagine what a nightmare it would be to suddenly have everyone in the park crowding those walkways headed towards one destination at once.
Overall, I think this may just be wishful thinking. A lagoon show doesn't make sense.
