The "gravel" or "pebbles" is supposed to rubble, if you'll notice the squarish shape of the rubble areas. I plan to create some broken columns and other structures more than just a pile of pebbles, but this is the first step.

Regarding the kelp, it can grow to over one hundred feet in length with each "leaf" growing several feet from the stem. So is the kelp too big, really, or do I need to place some other form of scale in the map, so you can get the feel that this isn't a miniatures scene.

I actually put a very subtle caustics transparent layer on top of the whole thing, I increased the transparency, as the caustics caused a kind of blurring to some of the image at lower levels of transparency setting.

Again, I'm only in the experimental stages at the moment.