Is seawater pure and can be used for ablution? Is it permissible to urinate in the sea water or in a swimming pool?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam,  is His slave and Messenger.

Seawater is intrinsically pure and purifying for other impurities; urinating in the sea is permissible for it does not render seawater impure and unusable. As for urinating in a swimming pool, if it is approximately as big as a lake and urinating in it would not change its properties or render it repulsive for others to use this water, then it is permissible. However, if the swimming pool is not that big, it is forbidden to urinate in it, whether the water is equal to the known standard of two pitchers, i.e. approximately 160.5 liters, or not.

This is the opinion of the Shaafi‘i and Hanbali schools of Fiqh (Islamic jurisprudence), whereas the opinion of the Hanafi and Maaliki schools of Fiqh is that urinating in the small bodies of still water is forbidden; for it contaminates the water, rendering it impure and worthless.

However, the Shaafi‘i and Hanbali schools of Fiqh do not consider urinating into the little amount of still water that might spoil it  prohibited; for water is not a customary source of income. Moreover, it could be easily purified by adding extra water to it. Undoubtedly, it is safer and more prudent to adopt the opinion of the Hanafi and Maaliki schools of Fiqh in this regard.

Allaah Knows best.