Kennedy, I suspect that the introduction of a material to the Still Water upset the uniformity of the curing and drying process, and thus resulted in the cracks. You could try a different technique in the future, such as a prepackaged mud product such as Vallejo's Dark Earth, or formulate your own mud using white glue and common backyard dirt. Once you have a muddy base that is to your liking in texture, form, and color, you can add the Still Water untinted to give the pooling and glistening effect that suggests a very muddy environment. However, adding color to Still Water should still be doable, as seen in the YouTube clip "How to use Still Water with vallejo colors." Lastly, because the Still Water product readily accepts repeated layers, hopefully any cracks on your diorama can be covered, smoothed over, and hidden by a fresh, thin layer, of the product.