The state does not have specific laws to such action. The legal assumption is that the authorized person/family member will use a method that is respectful, adherent to the wishes of the deceased (if directions were not included in a will), pursuant to the existing health codes and will not create a public disruption (spreading ashes in a public place).
Answer:
Spreading the ashes would constitute waste disposal so they cannot be spread in water or on land adjacent to a water course. Apparently Disneyland has quite a problem with ashes being dumped into the exhibit areas.
Alternately the ashes can be: