I would expect sea landings are not part of flight planning apart from the life jacket and evac briefings. While SYD-JNB flights operate over remote areas, so do SYD-SCL, HKG-JNB and SYD-DFW. Recent sea landings in mid flight have been catastrophic so not really worth considering (think AF, SA, Swiss)

VA withdrew from MEL-JNB because their 773 had to detour north of the great circle route and this incurred a 2 hr additional flight time (for a shorter Great Circle distance) compared with SYD-JNB which QF operated by 744. This was so the VA 773 could operate within approved ETOPS limits. The 744 had no ETOPS limits because such limits do not apply to 4 engine aircraft. Ie, VA could make better money elsewhere with more efficient aircraft utilisation.

Fight tracks depend on winds which vary at altitude. I have been on QF JNB-SYD that crossed the Australian coast some hundreds of Km north of PER. Just see how far that is north of the Great Circle Route!