Is seawater purifying? How about water from a river? Are there any general rules for determinin

In the Name of Allah, Most Merciful and Compassionate

Both sea water and river water are purifying, meaning that it is valid to use them to lift ritual impurity (through wudu or ghusl) and also valid to use them to remove filth.

Water as it comes from nature is purifying, no matter what quality it may have. It ceases to become purifying when (1) it is used to remove ritual impurity or remove filth, or (2) when it changes through admixture with something pure such that it is no longer termed water, or (3) when it is affected by filth.

For further details, please refer to The Reliance of the Traveller, section e1.

And Allah knows best.

Hamza.