Learn Spanish (Latin America): Como la arena está seca, ando humedeciéndola con el balde. - Since the sand is dry, I go around dampening it with the bucket.

Using the verb andar instead of estar with a gerund (the -iendo or -ando form) creates a diffuse or intermittent progressive action. While estoy humedeciéndola simply means "I am dampening it" right now, ando humedeciéndola gives the feeling of "going around" doing the action, doing it bit by bit, or doing it over an extended area (like moving around a sandbox with a bucket).

Could I start this sentence with Porque instead of Como to say "Because"?

No, you generally cannot start a Spanish sentence with porque to mean "Because [this happens], [that happens]." When the reason comes at the very beginning of the sentence, you must use Como. If you prefer to use porque, it has to go in the middle: Ando humedeciéndola con el balde porque la arena está seca.

Why is there an accent mark on humedeciéndola?

When you attach an object pronoun like la to the end of a gerund (humedeciendo), you add an extra syllable to the word. To keep the natural spoken stress on the same syllable it had before (the e in cien), you are required to add a written accent mark.

Why use está seca rather than es seca? Isn't sand naturally dry?