in the video that was posted by ncc2015 he guy coats his acrylic with a sealant and then again with silicone.... I'm not sure if both or even either individually are necessary. his are for a waterfall and then a pond that will contain the water so I'm assuming that is why he did it.
Later today I am going to finish the rock waterfall i made from foam, acrylic tinted drylok, and some acrylic accents - I think I'll silicone the parts the water will run down (I carved a little stream into it) but not the rest. If it isn't necessary I would rather leave it looking as natural as possible - not shiny and glossy from silicone.