I once made an ocean scene where the water was blue tinted piping gel. I wanted to make breaking waves with foam and to do that I put a little buttercream on a toothpick and dabbed it into the wave. The wave then looked like it had foam on the top....you can see this on my margaritaville cake in my gallery if you look very closely
Anyway, I'm telling you all this because I think if you make the water coming out of the hose out of blue piping gel and then mixed in a little light blue buttercream (after its on the cake) you may get the look you want. Hope this helps.
Keep me posted....I'm curious to see how the cake comes out.