I'm not that familiar with the F! theatre, but if the lake is shallow enough you could probably get away with not using the water table (like the Bellagio show). The water table serves as a "fake bottom" to stabilize the fountains (Paradise Lagoon is 18 feet at its deepest), and it allows them to easily retract all the fountains below water level to hide it during the daytime (not a problem at DHS).
The better question is why anyone wants it in the F! theatre. Think of this from an Imagineering perspective. As anyone who has physically seen the show can tell you, the show is more than just the fountains. It utilizes the entire Paradise Pier infrastructure. The Ferris Wheel and Roller Coaster figure prominently. The atmosphere in the area before the show, with all the lollipop and popcorn lights reflecting across the water, the colorful boardwalk; all this is part of the theming. If they put it in the F! theatre, what is going to be the backdrop for the show? How is that gonna look as cool as what is at DCA?