When one is in a state of minor or major ritual impurity water is the only means through which one can achieve purification.
As for purification from impurities in general, water and other substances can be used. In general, whatever removes the filth whether it is water or any other substance is sufficient. There is a beneficial maxim which can be applied here regarding water:
All water that descends from the heavens or comes forth from the earth is pure in itself and purifies other things as well.
Rain water, whether it is running in valleys or has collected in an area is pure. A person can perform Wudhu and purify himself from major ritual impurities. This applies to sea water as well. The Prophet (peace and blessings be upon him) was asked about the sea and he (peace and blessings be upon him) said:
"Its water is pure and its dead(1) are lawful." (Abu Dawood).
It is clear from this Prophetic Tradition that sea water is pure and purifies one from minor and major ritual impurities and from impurities in general.
Dust or dry sand is also a purifying and cleansing agent.
Purification in this manner is called 'Tayammum' (dry ablution). Allah, the Exalted, says:
(And if you are sick or traveling, or if you have an intercourse and could not find water, then perform Tayammum using pure dust and wipe therewith your faces and hands.) (5:6)
1- This refers to the animals that live in the sea, not what has fallen into it and died thereafter! Therefore, if a sheep falls into the sea and dies as a result of drowning, it is unlawful for consumption.