My first thought is that the seaweed / kelp is too large in comparison to the model so it looks like a scale model at the bottom of a fishtank, I think the large pebble texture adds to this - (which is great if you're after a miniature model feel)....the seaweed itself has a lovely slightly fuzzy quality to it. You might want to put in some caustics as an overlay? - not sure it might be a little too cliched, but worth a try just to see how it looks.