Russia seems to be paranoid about a Ukrainian amphibious operation. They spent a lot of money and put a lot of effort into building beach fortifications in Crimea too.
Logistically, an amphibious operation was never in the cards for Ukraine. NATO allies have provided some amphibious craft, but only enough to conduct raids on Russian held territory in Kherson oblast. Ukraine does not have the amphibious craft to conduct a full blown amphibious assault.
Ukraine has been harassing the Russians in Kherson oblast and they may have been playing on Russian paranoia about amphibious operations to make them think that was a vector for the assault, but I'm 99% sure there was no plan for a large scale offensive from that direction unless the Russians completely pulled out of the region leaving the river free to build a pontoon bridge capable of getting heavy equipment across.
When making a river assault, the troops crossing the river are vulnerable as long as they are in the water. Crossing a narrow river is more doable with enough supporting fire to keep the enemy down until the first wave can get across and establish a beach head, but the wider the body of water, the more support fire you need. The US relied on battleships to support naval landings in WW II. Literally nobody has those types of guns anymore.
There are realistically only three places where the core offensive can come from, in the south west south of Zaporizhzhia, a bit further east in the south towards Mariupol or in the Donbas, most likely the north so they can move down the ridgelines.
Once the Dnipro flooding resides, the Dnipro will be at lower levels than before the breach and Ukraine would be faced with a more standard river crossing attack than crossing a large, wide river. Russia basically opened up the southern front to more vectors for attack, though they won't be open for a few weeks.
There are stories of Russian troops being swept away by the flood waters. Russia pretty much destroyed all their defenses on the left bank
Putin’s troops ‘swept away’ in flooding after dam destroyed, Ukraine says
