Dry floral arrangement foam, available from florists or craft stores, is an easy to carve materal which will hold wire landscaping materials easily. It can be finished with a coat of gesso to protect it from dents, and shapes can be adapted if your model changes in the future.
Straight polymer clay blades work well to make slices of foam for thinner display pieces. This koi pool was built in a piece of foam just under 1 inch thick. When finished it can be glued into a larger garden of similar pieces, or used to create a stand alone water feature.
If you need pieces of floral foam larger than the ones you can buy, cut the pieces to your desired thickness and glue them end to end with pva (white) glue. For larger scenes, glue all the pieces of floral foam to a solid base, then carve your landscape details, including water features, with regular sculpting tools and loops including ribbon tools and clay cleaning knives.
A pool I built earlier to 'house' my butterfly koi was easily expanded by gouging out more foam and rounding the shape into an existing round pool. The foam is so easily worked that I could enlarge the pool while the koi and pool bottom remained in place.