I think the center area is too dark. I would vary the color gradually from dark brown near the center to lighter brown (as you have) at the edges. You're trying to simulate a muddy bottom. You could add some fine brown ballast or real dirt to blend in the edges of the plaster cloth and even a bit of green ground cover. The roughness is realistic for a pond bottom and should look correct after you pour and the water surface levels out. Experiment by taking a paper plate, finish and paint it like the layout, then do a test pour. The Envirotex "water" on my layout is between 1/16" and 1/8" deep. I left it that way - just one pour - because the Envirotex seems to give the illusion of increased water depth. Make sure the boundary rocks are higher than the depth of your pour. I did this only once but it came out well.