I’m going to go ahead and posit a third option, which is that the passage is not intended to present an exhaustive account of those who survived or those who perished. Starting from 2 Peter 2:4—“For if God did not spare the angels when they sinned, but cast them into Hell and committed them to chains of deepest darkness to be kept until the judgement;”—Are we to understand that this refers to all of the angels? Why or why not?
2 Peter 2:5—“and if he did not spare the ancient world, even though he saved Noah, a herald of righteousness, with seven others, when he brought a flood on the world of the ungodly;”—‘he did not spare’ is the same phrase as was applied to ‘the angels’ above. It is hardly indicative of universal destruction.
2 Peter 2:6—“and if by turning the cities of Sodom and Gomorrah to ashes he condemned them to extinction and made them an example of what is coming to the ungodly;”—were the cities entirely turned into ashes, no ruined buildings standing around? Were all the people of them made extinct? We know quite well that they were not, because ‘extinction’ would mean no survivors and that was not the case.
2 Peter 2:7—“and if he rescued Lot, a righteous man greatly distressed by the licentiousness of the lawless 8(for that righteous man, living among them day after day, was tormented in his righteous soul by their lawless deeds that he saw and heard),”—leaving aside for the moment the righteousness of offering one’s daughters to appease a mob, we would think from this passage that Lot was the only survivor of the doomed cities, when we know that in fact his daughters also survived.
2 Peter 2:9—“then the Lord knows how to rescue the godly from trial, and to keep the unrighteous under punishment until the day of judgment 10—especially those who indulge their flesh in depraved lust, and who despise authority.” —Here we see that including Lot’s daughters may have been counterproductive to the point Peter was making, which was not, in fact, to exhaustively and accurately tally everyone involved in all three incidents and their fates, but to provide select examples of divine action: God has saved some and destroyed others.
1 Peter 3:20 is even easier to understand in this context: God saved eight people through water, which prefigured baptism. It says nothing about people who were not saved through water because their lands didn’t flood.