I agree the lake is under utilised. It's a big stage so you a really big show to fill the space. That's what I think good shows have in common, they're big shows in small spaces. That's why Pirates Unleashed was such a failure, it was a tiny show on a massive stage. Same is true for HSD V Police Academy, PA had more stuff going on crammed into a smaller space, so the show felt bigger. I think that's why the Ski Show works, it's a show that takes up the whole lake and only just fits in the space. Jet Stunt Extreme does a better job of filling the lake but it's not really a show, rather it's an extreme sports demonstration.
What I think they need is either improve JSE or go back to basics and put in either a classic Ski Show or a big new Stunt Show, as well as using the lake for a night time spectacular. There's no reason you can't do both, and with such a big venue so close to the entrance it's ideally suited to a big night show. Fountains would work fine as an element of a night show, but it shouldn't get in the way of the day show.