I've personally used and had decent results with sand mixed into epoxy resin.
You can usually find small kits of pour-on type epoxy resin at big hardware stores in the paint section, or craft stores in the sculpting/mold section.
just mix up a bit in a disposable container, and then add in some sand. application can be a bit messy, since you can't really brush it on, just glop it on and smear it around.
I usually would put a dusting of dry sand over top of it, and then rinse this off after it was cured, just to cut down on the gloss (wouldn't be an issue for an underwater background, but I was building a paludarium)
You could try brushing it on, and then just sprinkling sand over it, but it wouldn't be quite as durable, and would eventual get rubbed off - especially if you have any sort of rasping fish.