Is seawater pure for ablution, and is it permissible to urinate in it or in swimming pools?

Sea water is pure and purifying; urinating in it is not disliked and does not harm it.

As for a large pool of water (birkat al-sibahah) that reaches the extent of being like a vast body of water, such that the soul does not find it repulsive, urinating in it is not disliked. Otherwise, urinating in it is disliked. This is the view of the Shafi'is and Hanbalis.

As for the Hanafis and Malikis, they hold that urinating in still, small water is forbidden because it contaminates it, renders it financially worthless, and deceives others into using it. The more cautious approach is the view of the Hanafis and Malikis.