Different religions have their own view concerning ashes and cremated human remains and it recommended that if there are religious considerations that you contact the respective faith authorities to ensure compliance with the particular faith.
The following is a guide only to the considerations you may wish to investigate
Hindu
Cremation is fundamental to Hindu beliefs, the ritual is designed to do much more than dispose of the body it is intended to release the soul from its earthly existence. Hindus believe that cremation (compared to burial or outside disintegration) is most spiritually beneficial to the departed soul and is based on the belief that the astral body will linger as long as the physical body remains visible. If the body is not cremated: the soul remains nearby for days or months.
After cremation the ashes are not allowed to return home and the final act of scattering them must be within 14 days of the cremation. The ashes must be cast into a river, ideally the Ganges, and the mourners should then depart without looking back.
Sikh
Sikhism is distinct from Hinduism in many ways however funeral rituals are similar in that the ashes are strewn over water. Some families, living outside India, prefer to take the ashes to Punjab. The Ganges doesn't have the same status as it does in Hinduism.
Taoism
Taoist choose to bury the body, Taoist tradition considers
Buddhist
Theravada tradition says Buddhists cremate the body of the departed - in some Buddhist cultures burial is preferred if the parents of the deceased are still among the living. This is more of a cultural practice than one with any significant Buddhist meaning.
Buddhists believe that attachment towards the dead is detrimental to the progress of the soul in seeking a new life after death. It has been noted that scattering of the ashes could be seen as charitable act - not widely accepted yet.
Catholic Church
Although for a long time cremation was banned by the Catholic Church (lifted in 1963) but in 1966 Catholic Priests were allowed to officiate at cremation ceremonies (The Church still officially prefers the traditional interment of the deceased).
Cremation is now permitted providing it is not done to express a refusal to believe in the resurrection of the body. The Church does make it clear that it does not accept scattering of the ashes, the body is to be treated as a whole and should be kept as such,
Church of England
Church of England permits cremation. The matter of scattering cremated ashes is governed by Canon B 38.4(b), which provides as follows:
The ashes of a cremated body should be reverently disposed of by a minister in a churchyard or other burial ground in or on an area of land designated by the bishop for the purpose or at sea.
The ordinary position, therefore, is that ashes are to be buried. They may only be scattered if the bishop has designated land for the purpose of the disposal of cremated remains on that land/sea.
Confucian
Traditionally the culture was that the body should be buried in the earth to preserve its integrity. There is an old Chinese proverb that burying the body brings peace to the deceased and burying the body in earth is a way of showing respect and filial duty to the parents.
However this is cultural and not doctrinal.
Judaism
Traditionally Judaism disapproved of cremation in the past. However, during the 19th and early 20th centuries, as the Jewish cemeteries have become crowded and were/have running out of space, cremation became an approved means of corpse disposal amongst the Liberal Jews.
Reform Judaism still support cremation, although burial remains the preferred option.
The Orthodox Jews have maintained a stricter line on cremation, and disapprove of it as Halakha (Jewish law) forbids it. This halakhic concern is grounded in the upholding of bodily resurrection as a core belief of traditional Judaism, as opposed to ancient trends such as the Sadduccees who denied it.
Conservative Jewish groups also oppose cremation.
Islam
Islam categorically disapproves of cremation. Islam has specific rites for the treatment of the body after death. Usually, the deceased are bathed and then wrapped by a single piece of cloth for burial which takes place within 24 hours without a casket. The soul of the deceased cannot find a body to reincarnate itself in the next life if the body is not buried in the ground
Overseas
If you wish to scatter ashes abroad it is prudent to contact the embassy of the country you travelling too as a first step