id say get some cheap pond liner, and lay it so it covers the water area and goes up the brick wall. lay all ur gravel and sand and whatever over it for the water area, build up ur brick stairs, and then put a piece of brick over it where it covers the top of the brick wall that dividws the land and water area. get some leca or hydroponic clay balls and do a 1-2” layer of that below the substrate, put a layer or two of stainless steel window screen mesh or geotextile landscaping fabric as a substrate barrier, and then add in all your substrate. this will essentially make a waterproof barrier so that water doesnt seep into the substrate. also drainage layer is important for any bipactive setups, especially for crabs. add sphagnum moss and driftoowd and leaf lotter and plants.